First it's literally how they make money. Selling ads. Meta doesn't have any other real source of income. (RayBan glasses don't count.)

If they don't show you ads, they lose money. (Because they need to pay for the servers, traffic.)

Second. Lots of people say "I don't look at ads, I don't care, they don't work on me", but actual behaviour says otherwise.

Third some people just install uBlock to their families and they don't even know they have it.