I do not understand why solar fields do not take the ample roof areas. That is usually just wasted, even worse the roofs are mostly of some dark color where I live, they heat up the area for no good reason at all.
That's true, but a non-trivial factor driving down the cost of solar electricity is not only the panels themselves, but economy of scale in all the rest, like electrical systems to connect them to the grid, support structures for the panels, installation and maintenance, etc. etc. So rooftop solar, while attractive from the perspective of not industrializing the wilderness, is more expensive than utility-scale solar.
But yes, it would be nice with building codes requiring to either have solar or green roofs. That would certainly help drive down costs, and in the case of green roofs, reduce cooling demands as well as providing (depending on what you plant, granted) a haven for biodiversity.
The main impediment to roof-top solar is actually the roof tops themselves. Most were not constructed with the weight of solar panels in mind, nor are all roofs built to a common standard. Thus every roof needs an expensive inspection to see if it can have solar panels mounted as-is, and if not you also need an expensive custom plan to reinforce the roof.
Also the grid is really not designed for large amounts of decentralized power generation. While modifying the grid is probably easier than replacing roofs en masse, it's still expensive. Dedicated solar plants can take advantage of economies of scale.
But yes, it would be nice with building codes requiring to either have solar or green roofs. That would certainly help drive down costs, and in the case of green roofs, reduce cooling demands as well as providing (depending on what you plant, granted) a haven for biodiversity.