It's because of the First Past The Post voting system that is used in the US, which means winner takes all.
If you want to understand why the US seems so uniquely politically divided, then you will understand once you understand how FPTP voting works and how it inevitably leads to binary politics. Most countries have a voting system that leads to broader representation. US politics will always be black and white and divided so long as FPTP voting is used.
There's a great video by cpg grey explaining FPTP: https://youtu.be/s7tWHJfhiyo
FPTP is bad but other methods are similarly flawed:
https://www.youtube.com/watch?v=qf7ws2DF-zk
Keeping a democracy requires people to understand it as well. At least in US it seems they'd rather elect a dictator wannabe who clearly said he wanted to be dictator for a day than elect a woman.
Fortunately the US demonstrated they can elect racial minority president, so different gender president can happen too.
It's a problem in Canada as well. I believe FPTP's issues are exacerbated by modern developments like social media, but it's just a hunch
In most Anglo-Saxon places that inherited FPTP. Some places like Australia have made changes that make it better. It is possible.
We've consistently voted against this kind of progress in Canada for some reason, but I agree, it is possible.