There are a variety of motivations. These include...
The biggest is that they are trying to maximize revenue/profits. In controlling the flow of information to people they are in better control of that. A lot has been studied about this.
I didn't realize there were other motivations until FB started to publish studies they carried out on people. Some of those (which they published awhile ago) were around the manipulation of users emotions and their ability to target voting groups (to increase people going to vote in target groups) around elections.
Yeah I can think of a few. That sort of influence is one, benefiting advertisers is another, positioning themselves to get a cut of all transactions is another. Is there an alternate universe where in order to get your content in front of users, it has to be AMP and in order to be AMP, it has to meet a new requirement that all payments must be done through a payment framework that lets google control the user experience (“purely for the end users’ benefit” of course)? Probably.
I hear what you guys are saying, but "maximize revenue/profits" means "benefit advertisers so they come to us instead of anybody else." "Getting a cut of all transactions" means benefitting advertisers who sell anything through the site.
What? Getting a cut of transactions has nothing to do with advertisers. Are you saying apple forcing app payments through the App Store is about advertisers? Seems like you are trying to fit all scenarios to a predetermined narrative here.