The main reason: The bike store can fit more riders on fewer frame sizes, since the seat post has a large range of adjustment.
They probably also argue that the frame is stiffer, due to shorter stays and seat tube.

The stem sloping up is the shortest distance from the steerer to the bars, so it actually makes sense to do it that way.

I ride a modern Orbea, and love it. But, I rescued a 1970s Raleigh International from the trash. I do like it's look, with chrome lugs and skinny stays.