They can. But it would be very expensive to replace more than 20 years of domain experience.

It would probably be a lot more expensive to replace more than 20 years of domain experience immediately because you quit to go work at rand($FAANG).

Exactly this. If you are a single point of failure knowledge silo, the business sees you as a liability since if you decide to quit, their business is then screwed. This is why turning yourself into a silo is making yourself disposable - it gives the employer a very good reason to try and find an alternative to continuing their reliance on you.

I agree, meanwhile they prefer giving me a raise than start looking for backup or replacement

Why does one exclude the other?

short term, it cost more to bring a backup/replacement than give me a raise.