They must have a reason for that. I don’t think they really think they have a profitable business with this special affiliation and they are doing it for it. Sony chose to open it, but with the drives requirements, Microsoft chose not to open and control it.
With the Xbox 360, you can play games from an external HD drive and even a USB drive
With Xbox One (X), you can play games from a high-definition external drive and fast enough from a USB memory stick.
In fact, you can take your external hard drive to another Xbox console, log in there, and voila you’re carrying your games with you.
Given their previous actions that made this possible, it seems very unlikely to me that this was based on a choice of revenue generation through expansion, but rather a technical choice. I also wouldn’t be surprised if a future Series X update gets a standard port/option, when compatible SSDs are more popular/affordable.
Long story short: this mistrust is Microsoft’s motive which is in complete contrast to the number of opportunities they previously offered players to play games from their own medium. And if you don’t mind, you can still use an external HD to store games, but you have to transfer all the data to your internal SSD when you want to play. Which is still much faster than downloading everything again.
