Another point to consider is the IPv6 global routing table for internet routers. Suppose you decided to give end users /120s instead and those /120s were all routable on the internet. That means there are 120 bits of addressing just to find the network. There are 2^120 such networks in theory. If you could actually enumerate this, you'd be well on your way to bruteforcing AES128. In other words, this is just infeasible.

By handing out /48s the routing table stays manageable. This is the smallest address block you can announce via BGP for this reason.

Given the utter vastness of IPv6 we are also able to do things like carve out an entire /7, fc00::/7, for unique local addresses, and still tell people they shouldn't actually need these addresses at all.

As to the actual process of getting a PI block, I think it is likely to involve some questions. A similar objection exists to handing out smaller than /48s: more people having their own block implies more routing entries. Much better if an existing provider carves you a /48 out of their allocation and routes you traffic. This is probably balanced against the fact that by requiring you to deal with an existing LIR and set up BGP (and your LIR won't want you to muck that up) the number of people who will actually do this just for fun is limited.