Because companies have imperfect reading on worker productivity, and are also trying to balance other priorities when determining pay, high output workers are consistently underpaid, called Wage Compression (https://en.m.wikipedia.org/wiki/Wage_compression)
After getting a 2% raise in the last cycle, and knowing that my granted options were going to be worthless (a factor which is under-discussed in popular news articles), I decided that it was a particularly opportune time to jump. Like many, I was not particularly unhappy, so I had not been motivated to go through the effort for search process before.
When I got an offer, my employer quickly matched it (suggesting that I really was worth that much to them) but burning a bridge with the contact you got me the new job was not worth simply reaching parity in my current one, so they lost me. If they had preemptively gotten a few percentage points closer before I sunk effort into interviewing, I probably would not have started looking.