(1) Network effects. People want to use social media that everyone else is using. Once a site achieves a critical mass of users it becomes the obvious choice to join. It also becomes difficult to leave because if you have built up a personal network on most sites, you can’t take it with you.
(2) Convenience. Most sites don’t require a lot of effort to use. In the past few years this one has surprised me a bit. The level of effort most people are willing to put in to trying a new site is basically 0. Using something like lemmy requires you to read a few paragraphs and make a decision about a home instance. That is too much effort for a lot of people.
Sometimes this happens to me as well. Usually when a celebrity does something news worthy that is how I discover it. Or when sports things happen.
I will say though that, while you are probably right that only learning about news through memes is not a good way to be informed, the other extreme (obsessing over every headline) is probably bad too.
As unsolicited advice I’d recommend finding a few long form journalism sources and following them to get a better feel for the news. Doing the over time I have found that i am informed without the hysteria that often comes from 24 hour news cycles.