TL;DR: Staying in place is significantly more desirable to employees than leaving, hence less rewards required.
It is perfectly logical. It is due to three major factors. Loss aversion, momentum (or lack of it) and irrationality.
Loss aversion is the fact that we value what we can lose more than what we can gain. Google around for it, here is a fun video about it:
Momentum is just about comfort and direction. It is easier to continue what you are doing than to change, change takes energy.
Irrationality is about massive inequality between how we treat our relationship to a company (loyalty, late hours, etc) versus how the company generally treats us (replaceable widgets). We are irrational towards companies in the same way we are sometimes irrational towards objects (I love my “object”[car, teddy bear], or in my case, my ancient Popple:
The combination of these factors make employees easier to keep, and new ones harder to hire away from other company X. To leave requires you to get over loss aversion (what if someone finds out and I get fired, what if new job is worse or you get caught in last in, first out situation)… then you have to work up the energy and motivation to leave (and prior to that, do resume, do interviews)… cancelling out all that momentum… and then you have to convince yourself that you are not being disloyal to your current employer, leaving them in a bad spot or just being a “bad person” (protect your ego).
The right™ thing to do, I believe is to ALWAYS be looking for a better job, constantly and forever (this makes momentum work for you, not against you). Never be happy, never be settled, never let momentum hold you in place. Never consider a company your family, people can be awesome and keep in touch with them, but a company is soulless money making organization. Never feel guilty for a leaving a company, you have to look out for you and your family.
EDIT: Thought experiment (be honest). How much additional % would you require to leave your job today, no notice, no safety net. How much additional % would you need to stay and be “acceptably happy”. Now, be honest with yourself, then compare the two numbers and decide if you think they are rational.