Partially to help up sell those who have enough to go top tier but aren't necessarily dying to, or those who don't really have the funds but want the best anyway and will just figure that problem out after purchasing.
If you're a consumer who has the funds, who can say to themselves, 'well PS5 DE is only $100 more with added value', then they can also say, 'but then XBSX is also only $100 more than that with even more added value'.
Some consumers will say screw it and just stick with XBSS, and some will talk themselves into going with XBSX. MS wants to entice you to buy their product one way or another, and wants to try and make the XBSS and XBSX user base as equal as possible so pubs/devs can easily justify putting reasonably equal effort into both consoles capabilities.
I would say it is very anti-consumer to gimp your entry level beyond the technical capabilities just to make the higher end look better. And MS trying the cool guy attitude is very opposite of this kind of decision.
If XBSS is too good of a deal it's going to way outsell XBSX. That won't be good for XBSX non exclusive games in the future.
XBSS and XBSX are being set up like XB1S and XB1X from last gen. If you want the upgraded model, you pay more, but get a lot more.
XBSS isn't supposed to be the end all be all, it's a cheap entry point into next gen and the majority of casuals will be just fine with that.