It's lazy and stupid

Another way to say the above would be "simple and easy". Which is why it's done by a lot of small businesses that don't have the expertise (or the funds to hire expertise) to do something better

If it's a small town hardware store, it's easier for them to manage a Facebook page that they can access using their regular Facebook account.

Businesses are in the business of running their business, not worrying about FOSS principles and the open web. They can set up a quick information front without having to pay for a webmaster, hosting space, server space, an ISP to handle all that traffic, etc. So why would they care or want to spend the effort otherwise at their size?