Buy and sell hash rates? How this works? Why someone won't use them instead of selling them?

There are different reasons, here are a couple of them:

BTC Payouts - This allows for buyers to pay in BTC for hashrate. For example, you could be selling KawPow (Ravencoin) hashrate and receive payment in BTC instead of Ravencoin.
Algorithm Versatility - Buyers use the algorithm hashrate to mine different coins (like NeuroAI, Ravencoin, CLORE) and handle switching between these for sellers. This simplifies mining and ensures you're always selling to the highest bidder, which could mean mining the most profitable coin on that algorithm.
High Demand for Hashrate - Some buyers particularly seek out hashrate for reasons such as taking risks on solo mining. They may bid higher for available hashrate, resulting in better payouts than traditional pool mining.