Because they needed to sell hopes and dreams with a new system that has no new content
I mean, they basically took Sony's approach last gen, particularly their E3 2015 approach (those games didn't come out for like five whole years later). Difference is Sony was already building back some serious rapport with hardcore/core gamers thanks to 2011-2013 era PS3 1P releases, and early PS4 1P successes like Infamous: Second Son and Bloodborne (plus having the clearly more powerful system).
Microsoft is kind of odd because while they have consistent performers like Forza, racing games are somewhat niche and don't have the emotional/narrative hook a lot of people look for. FS 2020 is fantastic, but it's PC-only for now and again, no real story there for emotional/narrative hook. Halo's been in a rough spot for years; Gears 5 was solid but basically "more of the same", and they did fuck-all for Hivebusters promotion outside of a blog post talking about Bautista being in there.
Games like Ori and Cuphead don't even feel strongly associated with Xbox brand despite starting there, because they quickly went to PC and consoles like Switch (Ori) & PS4 (Cuphead). Minecraft is arguably the largest IP in gaming but barely associated with Xbox in any strong sense feels like. Games like Ryse and Sunset Overdrive are good but get overshadowed plus never got follow-ups to build off momentum. TitanFall never really built the community it needed and the sequel flopped; interest has picked up again but not in a way that's strongly associating the brand with Xbox. And then of course there were duds like ReCore and Crackdown 3.
So basically Microsoft has been a lot more scattershot on that front but they need to be much more consistent. Especially considering that while they may technically have the more powerful system, currently the best-looking games for current-gen are PS5 exclusives (Demon's Souls remake, Rachet & Clank: Rift Apart, Returnal etc.), and 3P releases as a whole have a (slimming, but still somewhat present) performance advantage on Sony's system for quite a few games.
Those are probably reasons why, even though Sony didn't have a lot going on from 2013-2015 in the 1P front (also can't forget the disasters that were Drive Club and The Order 1886...well, they launched in a poor state but at least Drive Club was improved with updates. The Order was forever a short game with mostly QTEs for its gameplay, quite shallow), they were able to push through that perception thanks to breakouts like Bloodborne, having worldwide presence and clearly better-performing 3P games.
Microsoft doesn't quite enjoy some of those benefits yet, not until they get some consistent 1P games, and 3P games start clearly and consistently performing better on their system. There's good chance they'll get there in the next few months, though, at least for some things.
