Incumbents often maintain large budgets to ward off competition. Competition wins enough to keep bidding, etc. If America is not full of monopolies, the bidding environment stays liquid. There is also the threat of using literal brand name targeting, or conquesting, which encourages your competition to run their own ads to keep you out.

I believe there are some agreements for larger corporations to continue to promote things even if it is not necessary. People with funny money can treat ad markets like billboards.

Another fucked up aspect is that the government often runs ads to promote its own policies.

Much like real markets there is sometimes unnatural activity that keeps it moving.

If you are like many of the commenters on here who is using adblockers and abstains from social networks, this is precisely why you see shitty ads.

Also if you aren't buying anything by clicking them, advertisers are ignoring you because you aren't a valuable user to them.

You are most likely reaping what you sew. Which shows that the ad networks are actually working.