Stalls are done as part of the certification process for a new type, they used to also be done in house after heavy checks before putting the airplane online. The "hold [pilots] by their hands" and "don't do anything 'risky'" mentality that has taken over at most airlines today prevents any real flying or knowledge of the airplane outside of the normal flight envelope from occurring/developing.
As a side note, at my carrier we train to a full stall (with break) in the simulator. It's not part of checking, but it is part of recurrent training.
Probably because the flight control system is supposed to keep the plane from stalling in the first place
Well the engines aren't supposed to quit, we're not supposed to bank past 30, cabins not supposed to go above 10K etc. etc., but we train/demonstrate all of that (not necessarily check). Any carrier that doesn't train to a full stall and any aircraft that hasn't been stalled as part of certification should not be in the air.