It's a bit messy for the employer. You can't just hand out 20% raises every time someone threatens to leave. Then everyone would be threatening to leave. And that's a hefty cost to add to what's likely your largest operating expense. Also, that's not just 20% in the employee's pocket, there are additional costs like unemployment insurance and the like.
OTOH, unless your employee plain sucks or the job is simple, it's almost always better to keep them than train a replacement. Tribal knowledge is valuable knowledge.
And no, only very small-time employers expect loyalty. They understand the game, and we should as well.
Funny that lemmy whines and moans about capitalism all day, without realizing they can play as well. Jumping jobs over the last 11 years got me $14 > $22 > $39. Been at this place 5-years, thinking about jumping ship again. Probably put me over $100K with a little luck. Oh, and I've never had such fat benefits or worked less. From home to boot.
Related: When we first moved here, a friend started at an oil change place, well below his skill set and previous pay. Kept job hopping and stacking his resume, now he's the top service manager at the largest auto dealer group. He quit moving, guess he's fat and happy. Sure drags in the $.