Umm, because it's the rules.
I was thinking about a dwarf with elven accuracy, and a careful reading of what Tasha's allowed you to change did not unlock the racial feats to other races. You mix in just randomly unbarring those feats, making it pretty obvious that your understanding of what is allowed is a lot more promiscuous then the rules actually say.
Please, go back and give me a direct quote from the rules that support that Custom Lineage can be defined as a specific race. I gave you direct rules quotes, if you can't do the same then it's pretty obvious I have the right of the rules. You are welcome to, as always, play them differently at your table with the DM's approval. But if you can't find any rule support to make it at least contradicting itself, then it's unambiguous in the wording that it's not any of the races.