Why not both? Just kidding!
It is almost certainly the lack of "flight risk". I've been on the management side long enough to know that you reward the most useful folks who "could go anywhere".
As GP said, it's a rational strategy for them. I personally have lowered my expectations of salary growth over the years, and would be happy with some of those other benefits GP listed in the mean-time.
I'd be happy if I were successful in my job and maybe had another promotion. It's been a struggle just to keep a Dev 2 job. No chance of promotion based on how "inconsistent" I am. I'm hoping not to get PIP'd.