The best of their pianos go to the concert stages and recording studios.
The next best go to their top clients.
The next best get reserved for their artists.
The next best get put into stores for "normal" people to buy.
Steinway, owned by a finance firm on Wall Street, has various quality control issues. They'll ship a six-figure piano with saw dust in it, lousy regulation, etc. Technicians know how to clean these up and work with them, but they don't hold a candle to e.g. the quality control of Fazioli. Steinway is the household name, and is ubiquitous in professional settings, so they can get away with it.