This may sound like a leap at first but it’s about building moats around the castle.
As an example: Gmail works best in Chrome. Having people use Chrome then keeps people using Gmail. Same goes for YouTube.
Vendors with something to loose give away browsers (or whatever) to keep people visiting/using their websites (or services). Once people are locked in they tend to keep using those same services rather than trying something new.
And then Vimeo and Hotmail wither on the vine while Google’s castle is hustlin and bustlin.
Companies are very protective of their websites.
As another example, when I post something good directly on Facebook I’ll get tons of enagagement. If I post it here then link to it from FB, I might see a quarter or maybe even zero engagement on Facebook. I suspicion, since the latter contains a link away from FB, they tend not to show that post as much to my friends and followers.
Facebook didn’t do away with chronological timelines because that’s what people wanted. They did it so they could control the flow of information. Just like Google, they want people to stay within their castle.
This topic also speaks to why every social media site has a free stand-alone mobile app. Click a link to a third party website? They keep you in the app instead of open the default web browser …and that makes it much harder for you to leave.