Because they think they have figured out a "magic bullet" to get into medical school, then realise it makes no difference. Also some schools require students start with 4 A-levels.
Medical schools are extremely transparent about their admissions policies, as any lack of transparency is immediately seized upon as potentially discriminatory to applicants who are not aware of such "hidden knowledge". To avoid this, they make sure to make it absolutely clear what they are looking for.
Additionally, the subjects taken are just a tick box in the medical admissions process - they check you have the required subjects, if you do then you tick that box and they continue assessing the other parts of the application. They end up with a lot of information and data from all applicants to decide upon so something as arbitrary as whether they're taking maths or physics is an unnecessary point at which to distinguish between applicants compared to e.g. UCAT/BMAT scores and interview performance.
Cambridge just require it because their course heavily emphasises the basic sciences and in third year all Cambridge medics have to do the third year of one of the natural sciences course options, and since all natsci students have to do three sciences/maths subjects, the medics would be at a disadvantage for that year if they didn't have an equivalent background. Also some of the second year medicine lectures I think are shared with some of the natsci papers available I think? But in any case, that is why they are the exception.