Many other countries including UK enforce a similar rule. It's very inconvenient in those countries, but there's a significant difference: in most other countries that have this kind of policy, visas can typically be processed in a timely fashion (and are actually processed at all). It's insanely expensive and very arduous administratively to get a visa for the UK as the spouse of a British citizen, but the process will typically only take a month or so.
> It's insanely expensive and very arduous administratively to get a visa for the UK as the spouse of a British citizen
How expensive is it?
Isn't the Uk the opposite? There are many visas in which you have to be in the UK to apply. This is why we have people coming on boats, and why they are not illegal immigrants. They technically have to travel here to apply for aslyum, and since they do not have a visa cannot take conventional transport, but it is entirely legal for them to come here on a small boat as long as they present themselves to the authorities to claim aslyum upon arrival.
Graduate visa's are the same for example, where you cannot apply abroad, so you must be careful not to leave the country between graduating and getting that visa.
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.
Asylum is an international concept negotiated by treaty. You apply when you arrive - that's true everywhere.