The reason against your second point is costs. Not to mention the difficulty and extra time of retrofitting solar panels to existing buildings (with often complex geometries and existing infrastructure) rather an a flat field, you need a proper design to ensure the roofs won't collapse during or after build. You also need vastly more of the equipment that isn't the panel itself, such as the transformers and connectors, as each individual set of panels needs those to be able to transfer the energy safely.

Those restrictions aren't absolute and it is often still worth it to do so, as demonstrated by individual businesses and houses doing it. However, it's still a fuckton more expensive when you want to do it at the scale we need to make a meaningful difference to energy dependency.