Ancient Elven Birth Rates [spoilers all]

Okay, so I know I've thought way too deep into this, but I was thinking about how if ancient elves were immortal, why wouldn't they just reproduce to the point where there basically wasn't enough room on the planet for them anymore?

Even if ancient elves did go into Uthenera when they felt done with life, they still lived a really long time before that and they still used limited resources and took up space (although if they could create pocket dimensions, space may not have been a big deal) https://100001.onl/  https://1921681254.mx/  .

Is it because all elves are born from spirits and sex isn't for reproduction? If that's the case, how did that come to be from an evolutionary standpoint? We know they have sex or something like it (see Codex Entry - Vir Dirthara: Birds of Fancy). It must have served some sort of function at some point.

If they are in fact capable of sexual reproduction, does that mean that have some sort of culture against having children? Or perhaps they're only fertile for a very short time.

My favorite idea, and undoubtedly the darkest, is that the death rate was so high (likely due to the Evanuris) that it still somehow managed to balance out the birth rate.