The conditions mentioned are pretty spot on in regards to flying frequency. Late finish before day off? Most likely. However, if your rest extends into your day off you are allowed that much time before your flight on your next day back. A lot of flying, within legal limits but not much more. Early start after week off? Most likely. Hell even early start after day off or double off within the limits mentioned above. It is rare to see a Captain's schedule empty (i.e. no flight, no off) for a certain day.
Inhumane? Well considering they have a schedule to keep and they adhere to legal limitations in regards to rest and frequency (6 on max) I would choose to call it intense rather than inhumane. With the latest surge in hiring as well as upgrades for Turkish pilots one hopes that it will only get less intense.
Why pilots leave? Foreign pilots more often than not are there for the time building and consider it a hopping stone to their target/dream job. Turkish pilots are mostly lured by the better pay and less intense workload (not to mention perceived prestige of working) at Turkish Airlines.
That being said, Pegasus are apparently hiring (locally and internationally) constantly as the airline expands and the expected 100 Airbus fleet starts arriving.