If FPTP is what prevent the US from having a true multi-party system, how come the UK got more than 2 parties?
Because FPTP
tends to result in a two party system
given enough time.
It's not like implementing one suddenly, magically makes third parties poof out of existence. Also, even in an already two party system, there would be expected transitions where parties "flip" or bifurcate into new identities relatively reapidly, which might allow ingress by third parties more than usual for a bit. This has happened several times in U.S. history. The FPTP is only supposed to be a loose influence pushing you gently toward 2 parties, not ironclad law.
Diagram nicely showing a 2 party trend, but by no means law, over a very short period of time:
Theres value in reps representing a certain number of people directly as their, specific, constituents.
Can you be more specific as to what that value might be, if they aren't in the same location? Traditionally, the value comes from things like "There's a steel plant in my district, I'm gonna look out for steel workers" etc. So local communities get their voices heard. But that falls apart if every rep has 35 steelworkers split up from the same plant in random scatter, as they become a tiny minority in every district, and get ignored completely. Only STATE-scale communities get attention from each representative, so you might as well just have everybody vote for all state representatives.