Bikes are hung upside down because the wheels are on the bottom But really imagine lifting your bike to the ceiling hooks rightside up, meaning you are lifting the bike by its spinning wheels. That would not be an easy task. Also spacing between the hooks is less important and can accommodate a larger range of bike sizes when hanging by the wheels.

They make bike hoist pulley systems you've probably seen but that can get pricey if hanging a lot of bikes.

You can get quite a few bikes into a small space if you stagger them and alternate direction so their handlebars aren't colliding and pedals/cranks aren't lined up. Too close and it's kind of a pain to snag one out of the center.