How much should you pay for that endorsement? Normally, you'd pay based on how many people see the ad. That, then incentives lying about how many people saw the ad, or creating fake views with bots. To deal with that, you add in Javascript that checks if the ad is genuinely visible and, try to validate that they aren't a bot, and whatever other fakery they try.

Adding a layer on top of that, people generally have a target audience, so if you want them as a customer, you might have to inject some more Javascript to determine if the user is part of the target audience.

And pretty soon the site is full of tracking beacons and blobs of JS.