The asylum system and immigration system are surprisingly disconnected from each other in the UK.

Pretty much all forms of permission to stay in the UK other than asylum can only be granted from within the country if you hold an existing long term status. So if you're visiting as a tourist you can't then decide to apply for a spouse visa or even a working holiday or student visa without leaving the country first. If you're already on a student visa or a work visa or similar you can change categories without having to leave.

The graduate visa is essentially an extension to the student visa with slightly different permissions - it makes sense that you can only apply to extend if you're in country and you view it from that lens.

The historic reason behind all this is that there used to be a substantial difference between being granted "leave to enter" and "leave to remain" (out of country vs in country applications). Leave to enter used to be granted by embassies etc and the foreign office, but leave to remain was granted by the home office. Now the home office handles everything in the UK centrally so the distinction is not significant.