Cat 3b approach decision height is 50 feet, and there isn't enough time for the pilot to transition from seeing nothing, to taking in enough of the outside world (and there won't exactly be much) to be able to assimilate it and land with reference to it.
This is why Cat 3 approaches have to end in autolands. Typically the FO is designated the handling pilot down to decision height, at which point he calls "decide". The Capt. has been looking out front since 150 feet, and if, at "decide" he can see the relative position of the runway is appropriate, he calls "land" and lets it do its thing.
There is nowhere near enough to see outside to be able to establish the required depth perception to flare. All that can be seen is a few runway lights, so the "land" call is in effect simply a confirmation that the aircraft's flightpath, which it has been flying with monitored accuracy all the way down, is actually going to land the aircraft in the right place. It may be a meter or so left or right, but thats good enough.
Although to continue the approach below 1000 feet there must have been at least 150 meters visibility (RVR), any reported vis reduction after this point is advisory, and the approach continues. The forward visibility required to avoid a go-around is to see ONE runway light. Bearing in mind the centreline lights are 15 meters apart, you can appreciate that a manual landing is impractical, and how little is going to visible out there.
Even after touchdown, when the captain has assumed control, the FO monitors localiser deviation and calls "left" or "right" if appropriate. Taxiing must be done at a snail's pace.
Cat 2 approaches have to be flown by the autopilot, and can conclude with a manual landing, and decision height is typically 100 feet above the surface. The required visibility (RVR) at 1000 feet is 300 meters, and to land the pilot must see 3 centreline lights plus a cross-bar, the latter required to give roll reference. There is enough time from 100 feet to take all this in and make the shift from instruments to looking outside (again the FO is handling, with automatics in, down to decision height).
Success at landing from seeing the runway at 100 feet requires (for me, at least!) a concious effort NOT to change anything until the flare. The aircraft has already trimmed itself, but the instinctive reaction on visual contact is to pitch up slightly. It is this reaction which must be avoided.
To flare, most pilots need to see a point ahead which has little or no relative movement. This reference point is therefore as far ahead as possible (this is why we teach student pilots to look at the far end of the runway during their flare. Their instinct is often to look at the runway immediately ahead of the aircraft. No depth perception! Crunch!). A more experienced aviator can pull it off without so much forward vis., which is why on a Cat 2, it can be landed manually with hardly any vis.
Regards - Musang