They have considered this, I think Facebook was even experimenting with it. But it isn’t as obvious a play as it looks.

1. Most people won’t pay, so you are spending resources creating a product that will be used by a tiny fraction of users.
2. The people who are willing to pay are typically those that you make the most off of with ads, because they tend to have more disposable income. This means that the price is much higher than you would expect.