Ask HN: Advertising companies keep contacting me. Why are there so many of them?

I have a somewhat popular website and no day goes by without an ad company contacting me, wanting to put their JavaScript on my site which then displays ads to my users.

Do all these companies have a reason to exist? Should I look into their offerings? Currently, I only have Adsense on my website. If they can really multiply my ad revenue as they claim, why don't they just book ad space on my sites via AdSense and pocket the difference?