I don't think they're a lot of surface parking on Switzerland like in the US

I do occasionally see parking lots with solar here in LA! So it is happening in some places.

You need some serious concrete and/or steel hardware to build a carpark roof that can hold the solar panels without easily being damaged by cars or broken by strong wind, so that massively inflates the costs. If you had a state owned company producing cheap solutions for this it could work though.

Every parking lot needs approval of the location, its probably a a pain in the ass, and would disrupt parking while being built which impacts sales (or will be perceived to anyway). If this worked, you only need to deal with a small group of people for a very large space.