Because it's accepted all over the internet and is the only way to pay on many websites.

It's also a way to send money to friends, just like venmo and such. And just like venmo and such, has very few consumer protections or regulation.

The US doesn't have a national or government system to freely send payments to other people - and no one uses checks anymore - so we all have to pick one of these equally awful private companies to use to send money and hope we don't get scammed or have our account frozen and our money seized.