Technically, if you own a fridge, you actually do have a heat pump. And it is heating your house slightly. But using heat pumps for heating has not been that common until fairly recently. So 13% is actually pretty good.

Using fossil fuels for heating has historically been quite cheap until that started changing a few years ago. For the same reason, building codes in the US are kind of sloppy when it comes to e.g. insulating houses properly. You can save quite a bit of energy if you fix that but until recently energy was cheap enough that a lot of people could not be bothered to care about that. Running the AC in a leaky house with single pane glass is kind of insane if you think about it. Yet, that seems to be standard practice in a lot of places in the US.