Is it different in the US to the UK? Surely you own the house and have a liability on the mortgage?
When we bought our house in the UK (a long time ago), it was a condition of the mortgage that we had buildings insurance. The theory is that if the house burns down or similar, the bank will want the rest of their money back and the house buyer is unlikely to be able to afford that considering that they needed a mortgage in the first place.
It's basically the bank just outsourcing a lot of risk to the insurance company (via the house buyer).
Why would they go via the house buyer? They can insure the house themselves.
It's common for the house buyer to want extra insurance (e.g. contents) whereas the bank is only interested in the house as a sellable structure, so it makes sense for the buyer to take on the insurance requirement (it's also less paperwork for the bank).
Insuring the contents of a home is routinely done as an entirely separate matter from insuring the structure. All renters have to do it that way. You can do it that way in a rent-to-own scheme too.
We've got combined buildings and contents insurance, but yes they're often separate. My point is that owners want more from the building insurance than a bank cares about.