It may seem that way at first, but space leaks during individual requests are way too easy otherwise. Think about something as simple as, say, parsing Mustache templates. The parser is probably internally making lots of little substrings to record the names of directives as it comes to them. You would think that this is O(1) in space, since you only need one (or a small number) of strings at a time, but without a GC the space requirements balloon to O(size of input). Being able to dump the memory at the end of a request doesn't help you if you keep blowing out your cache during each one.

I speak from experience: there have been many attempts to mandatorily delay collection until the end of the event loop in browsers, to achieve greater responsiveness. It has always failed. You can add heuristics to try to preferentially collect at the end of the event loop, but you really need to be able to GC anytime.