What is it about these 3rd and 4th edition books they have already done that you feel are not usable for 5th edition?
For a lot of people, the idea of buying books from a version to the game they aren’t going to play, so that they can then do the work of translating its contents into the game they are going to play, is just untenable.
That said, I’ve found that translating 4e monsters to 5e, with very little change other than a handy table for what numbers are normal at a given tier of play in 5e, leads to more interesting 5e combats. Magic items are harder in a lot of cases, but once you have a knack for reading what “X squares” or “Y Damage” translate to in 5e, it’s not too bad.
For me, I don’t trust the balance of anything from 3e, on any level. Mechanically/in terms of balance, my view is that 3/.5e dnd is mostly garbage. So, all 3e books are good for, for me, is lore and basic concept inspiration.
Either way, stuff like playable satyrs and dryads, elementalist (not wild magic!) sorcerers, hard-focus summoners, djinn pact warlocks, and all kinds of other stuff, are things that if they exist in old edition books, aren’t going to translate in a quick and easy way that a DM that isn’t super experienced is going to be certain of the comparative power level.
Races don’t impact the game as much as a lot of folks act like, so as long as you aren’t throwing several skill trainings or freebie spells, any race that looks like a phb race is going to be fine, but feats, spells, magic items, and class options are harder to balance, sometimes been for experienced DMs.
Also, not all groups/DMs like using homebrew player options. They aren’t “wrong”, and there’s nothing wrong with them hoping for more official options.