It's about economy of scale, which modern corporations have demonstrated that they are incredibly effective at. Bank fees that are a small fraction of each user's transaction but add up to billions for the bank. "First month free" subscriptions that only require you to remember to unsubscribe before they start charging.

When you have a billion users, than harnessing even a tiny fraction of their resources — financial, labor, cognitive, whatever — is a huge amount.

If our phones and social media apps take 2% of our attention away, that's next to nothing, but when you scale it up to millions of users, you're talking thousands of person-minds completely hijacked. It's like our brains are all running low priority threads bitcoining mining for these companies and getting next to nothing in return.