Biologists need a word to describe the genitalia and the reproductive roles of organisms. They have used sex for centuries and changing it isn't practical and probably wouldn't work anyway.

They would have to change every single text book and it would create confusion between people who have always used sex and those who would be taught something else. That's just off the top of my head, there would undoubtedly be other issues as well.

Then they would still face pressure from activists who would demand that the definition of the new word be changed to include trans people (e.g. "biological sex" was used for a short time to describe what reproductive organs a person had but some activists have just challenged them by saying that trans-women and trans-men are biologically female and male).

Considering that we have gender, sexual identity, gender identity etc, I don't know why anyone is demanding that biologists change the definition of sex when the current definition is genuinely needed by them.