This excerpt from the FAA Instrument Flying Handbook may partly explain that behaviour:
Haze
Atmospheric haze can create an illusion of being at a greater distance and height from the runway. As a result, the pilot has a tendency to be low on the approach. Conversely, extremely clear air (clear bright conditions of a high attitude airport) can give the pilot the illusion of being closer than he or she actually is, resulting in a high approach that may cause an overshoot or go around. The diffusion of light due to water particles on the windshield can adversely affect depth perception. The lights and terrain features normally used to gauge height during landing become less effective for the pilot.Fog
Flying into fog can create an illusion of pitching up. Pilots who do not recognize this illusion often steepen the approach quite abruptly.
It’s probably a combination of those physiological factors and the psychological factor of “Oh look! The ground. I’ve been hoping to see that for a while now, better not lose sight of it.” (All unconscious, of course.)
In answer to ortac’s earlier request this from FSIMS 8900.1
4-161 MINIMUM INSTRUMENT FLIGHT ALTITUDES. Except for certain CAT III operations, all instrument approach and landing operations have limitations related to obstacles, airborne instrumentation and equipment, ground-based navigation equipment, and/or visual aids. Because of these limitations, external visual information is required to safely complete instrument approaches and landings. Airborne instruments and equipment and the signals in space radiated by ground-based NAVAIDs must provide pilots adequate guidance to safely control an aircraft by reference solely to instruments until the aircraft arrives at a preestablished minimum height or altitude (DA/DH or MDA) for instrument flight. The total system (airborne and ground-based) does not provide this capability below the minimum height or altitude for instrument flight. Therefore, descent below the specified minimum height or altitude for instrument flight can only be safely accomplished when adequate external visual references are available. If adequate external visual references are not established, a pilot must execute an instrument missed approach at or before passing a preestablished MAP.
The FAA goes into quite a lot detail on subjects such as ALS and relation to minima, what visual cues are required at 200/100 feet, and protection of ILS systems during low IFR.
With respect to pirho’s point some airlines only use CAT II/III instrument guidance when the ground based controls required for CAT II/III are operative. If the area is not protected because CAT II/III is not in operation, a random van driver might result in the AP suddenly deciding to depart from a stabilised coupled approach below CAT 1.
Again as our american cousins might say YMMV.
This incident shows what can happen if the protected area for an ILS is not protected. In this case it was a CAT I approach:
Robert,
Again, that document refers to “adequate”. My experience is that it safer to wait for “good”.
Rwy20, JFTAOD, that incident occurred way, way above minima and doesn’t especially speak to the issue below DA.
Timothy wrote:
Again, that document refers to “adequate”. My experience is that it safer to wait for “good”
… and my experience is that a CAT I GP below system minima may not even reach the dizzy heights of adequate. 
The sensible solution is to go-around at minima if you don’t have ‘adequate’ visual references.
FWIW, I have done several tests with the autopilot engaged as far down as I dared, in VMC, and it worked ok. One was at Norwich, some years ago, down to about 50ft.
I never saw anything horrible happen, and consequently assumed it would probably work all the way to the tarmac. However I can’t tell you what the GS flag was showing
Indeed there have been pilots who said (mostly privately) they fly all the way to the tarmac. I recall a couple of SR22 pilots and one former owner of a similar high-perf type (rare, so I won’t say what) with an STEC 55X, IIRC.
Another data point, possibly of dubious value given its source, was an IMCR instructor (the highly polished shoes and epaulettes type) I had back at Shoreham c. 2001, who spoke in glowing terms of the French postal service pilots who, he said, routinely flew an ILS in zero-zero as a standard nationally-approved procedure. But he didn’t say what they did below 200ft.
It would be useful to know what is the best procedure in an emergency, for a zero-zero ILS. I suppose I could install a KRA10A and pick a CAT3 airport 
Incidentally:
If I got visual at 200ft then I might not disconnect immediately but disconnect maybe at 150ft, if the speed etc is all looking good.
One danger is that when one goes full-flaps at the cloudbase, immediately upon becoming visual, the plane baloons upwards unless one pitched forward with the yoke pretty decisively (which is what I do, obviously) and if you don’t do that, you could end up back in IMC.
Peter why change configuration at DA in an SEP with a long ILS runway? The best comments on this thread are if you are stable, and on the ILS and you acquire the ALS at DA just keep going. If you want to re configure then maybe on very short final at 100’?
I understand the crews flying Class D Jets have debates on when you acquire the PAPIs on a CAT 1, and whether the PAPIs protect your obstacle clearance, or whether you use the Glide Slope – and what about VASIs.
I agree it is better to do nothing at all at DA, with the exception of the argument about a really bad autopilot failure at 100ft which will probably kill you. A failure of a GA AP is capable of inverting the aircraft in seconds, but a human will never do that unless he has a stroke / heart attack / seizure / etc.
Peter wrote:
I agree it is better to do nothing at all at DA, with the exception of the argument about a really bad autopilot failure at 100ft which will probably kill you. A failure of a GA AP is capable of inverting the aircraft in seconds, but a human will never do that unless he has a stroke / heart attack / seizure / etc.
I doubt that. You will be monitoring the autopilot and have your hand on the yoke. Any attempt by the AP to do anything unreasonable will be noticed quickly.
