FPTP isn’t strictly necessary for districts, but it’s the most common. One way or another, you need some way to determine which candidate will ultimately represent a district. Unless you’re in a 2-party system, it’s very likely that this candidate will only represent a minority of voters in a district. Even with RCV you might get a “least disliked” candidate, but that’s still not a candidate that has majority support.

Perhaps to make it easier to understand: there is zero guarantee that all voters in a specific district have the same voting preference. And those without a plurality opinion are likely to end up marginalised under a districting system. If another group in your district is slightly larger, you end up without representation. Without districting, these voters can band together and choose their preferred candidate, without being constrained by arbitrary district lines.

Perhaps a concrete example will help. Take a random western country with a small minority. This minority doesn’t tend to aggregate in specific districts as much, they’re usually very well spread out over the country (let’s say there’s 2% nationwide, but at most 10% in any given district). Under a districting system, they’re likely to fail getting even a single representative, as they’re a minority in every single district. But under proportional representation, they could get a representative as collectively the minority is large enough to warrant representation with at least 2% of seats.

There’s also systems like the Danish, which iirc tries to figure out how many districts should be appointed to which party by dividing up the national vote (though I’m not very well acquainted with it). But even such a system will then be forced to assign a district representative to a district where the candidate does not enjoy majority support.

And that’s the issue with districting. It’s not possible to have a system that guarantees the national election results match the national voter preference, and that guarantees that district election results match the district voter preference.