Again, I don't know the laws in other countries, but in the US, it's generally relatively cheap to build your own powerwall with used electric car batteries. Total investment would be less than ~$2000 (United States dollars) though you would need to check your local laws to see if you're allowed to install it yourself or need a licensed electrician. Might be worth looking into.
This requires navigating complicated permits in most large US cities.
That process is expensive enough to make a purpose built option approximately equal in price. The companies selling this sort of system are pretty smart with pricing. Same for any kind of solar/water storage/etc solution.
No, you likely can not jerry rig this yourself and burn down the neighborhood.
Yeah that's likely not allowed, or really doable. Anything beyond a simple plug-in battery requires a licensed installer. There are some small capacity batteries that aren't that expensive, you just need the room for it.
A lot of houses here are terraced, so actual room for a powerwall is very limited. You usually only have a few meters of front and back exterior walls, with big windows. And any apartment owner is going to have to fight with the HoA to install something.
Not saying it's impossible for all houses. But I was replying to the "why don't we cover all buildings with solar" sentiment.