The saying goes if you’re not the customer, you’re the product. Now, you’re both. Every big social media app is testing subscriptions. It's a business model that comes with a perverse incentive. Just like airlines, if you make life worse, more people will pay.
Some run the servers with the expectation of donations. Some run them at a loss. Some of us selfhost.
There are all a variety of ways to keep Lemmy free, where as Reddit is hosted by Reddit. They decide everything about Reddit.