They regard it as key money. They keep it when the tenant has moved out so how does that keep good tenants?
If you pay the last two months the Landlord likely stiffs you, if you don't and sneak out you stiff the landlord for the utility bills - which is their justification for not accepting you not pay last two months in lieu of deposit return.
People saying they have always got their deposit back should share their secret. I got my deposit back once after a very nasty argument in which he accused me of changing the locks and when I told him that was BS and he knew it his reply was a "I pay poleet [so what are you going to do about it]" His wife's intervention was the reason I got it back.
Honest Landlords exist, but seem to be the minority. I don't doubt I will get my deposit back because I am dealing with high standard of conduct people, but that is so rare that it has hindered my moving.