Because HE makes them money. Trained pilots aren't exactly hanging out on street corners looking for work. They are rare and they are worth their weight in gold.
Really, you would ask the pilot to leave the pilots chair and put in a social worker to fly the plane ?
There are stories online, the phenomenon was called the "mile high club" where sex acts occurred between pilots and flight attendants, often as an initiation in the era before social media and HIV etc. In that era, flight attendants were hired based on appearance and the pilots could fire flight attendants if they wished to do so and there were no unions. Some retired flight attendants that I have seen, now geriatrics, were obviously very attractive when they were hired in their 20's. Sometimes passengers may have had sex with other passengers or flight attendants. The practice has died out now with increased CCTV and security measures. However the after flight parties and sex on stop over still occur within the flight crew.
lane: 
