Why not choose new words? Languages evolve in a complex way. One reason is that “they” is an easy choice. Another reason is that many speakers react harshly to unfamiliar pronouns, therefore it promotes acceptance to use familiar pronouns in new ways. I wrote with the Spivak pronouns for years, but that led to more distraction than understanding, so when “they” emerged as a standard, I adopted it.
One does not (always) do this. The singular “they” is many hundreds of years old.
If it confuses you, then I understand your confusion. Please read about the history of the singular “they” in order to resolve your confusion.
https://www.oed.com/discover/a-brief-history-of-singular-they
Why not choose new words? Languages evolve in a complex way. One reason is that “they” is an easy choice. Another reason is that many speakers react harshly to unfamiliar pronouns, therefore it promotes acceptance to use familiar pronouns in new ways. I wrote with the Spivak pronouns for years, but that led to more distraction than understanding, so when “they” emerged as a standard, I adopted it.