They dont have the advantage of owning the OS (android/chrome and iOS / Safari) nor the advantage of advertising on the google home page like chrome.
The "customers" they are courting are security/privacy zealots who constantly want them "just work on the browser" but never seem to have an answer on how to pay for that.
I am hoping the advent of LLMs allows more entrants into the space at a faster speed. Time will tell. Maybe as FF dies ladybird will take over.