Because it's a dual revenue stream. The retail customer pays you, and then the advertising customer pays you. Why make only $1 when you can make $2, $3, $4 over time?
If your next question is "why do they need to keep making more money?", the answer is capitalism.
When you get downvoted for making the obvious statement that you have to maximize profits as a capitalist entity, well, you know you’re in a venture capital forum.
If your next question is "why do they need to keep making more money?", the answer is capitalism.