Well cos actor was the original term (no women on stage in Ancient Greece/Tudor England). Then from 18th century actress was a term synonymous with loose morals/kept women and often literally prostitution.
Then in 20th century came the casting couch, fewer and lesser roles (leading lady but rarely the lead), a total dearth of roles after the studio system collapsed (they could barely scrape together 5 lead actress Oscar nominees in the 70s), plus the casting couch.
These days there is still the idea actresses can’t open films, they get consistently paid less, have shorter careers, far fewer roles, usually smaller roles, less developed characters, only play against men, cast alomgside men much older... plus me too.
So all in all it’s a term associated with being lesser, so most female actors would rather be called actors because they think it goes some way to moving away from all of the above.
The reason most people wouldn’t support loosing the male/female award split is because male actors still have far more roles and often better roles, so having separate awards helps the visibility of women in TV and film, so aids equality for now. Hopefully one day it won’t be needed.
All of this also true of theatre, though perhaps a little less..