Small businesses do this because it's free, quick to set up, and easy to maintain with no technical knowledge required.
Facebook obviously made it easy on purpose, hoping to create exactly this scenario - where half of all small-business presence is only on Facebook and if you want to see that content you need a Facebook account to do it.
I hate it passionately, but I absolutely understand how it came to be this way.
Small business owners aren't thinking about the ideological consequences of forcing people to engage with big tech to see their site, or that they should take a stand in promoting an open and decentralised Internet. They're just thinking "I need a web presence" and I guess I can't blame them for that as much as I dislike it.