If you feel like you have been treated well by your employer you wouldn't be looking to leave anyway.
People leave for various reasons. No matter how nice your employer may treat you, money often talks louder
Life also gets in the way. The last couple of resignations from my depot have been because people are relocating elsewhere in the company. A good friend at a other depot loves working for us but wants to move north to be with his partner.
We also lose a fair amount of people who move for promotions. Career minded people tend to move locations. We've also lost a few to Eurostar. The prestige of working for Eurostar is highly aspirational for some.
Even the best companies lose employees.