They absolutely are targeting the high end PC enthusiast with the Series X. They have said so as well but it's pretty evident. For one, which segment of the audience really truly cares about 4K/60/120? It's the PC enthusiastic that will spend thousands on a rig to play the latest games at 4K/60+fps. You've never heard a dedicated console gamer (i.e. someone who has never played played a game on a "high end" PC) complain about how they need 120fps for next gen

. Or how games are "lacking" if they are not 120, 144, or higher hz. Or how 1440p or 1800p is not enough

. Seriously, nobody outside the PC enthusiast crowd talks like that.
Second, let's not forget that Microsoft is building the "Xbox Series" as a new open platform that will cover the entire market. Series X is their high end and the Lockhart will be their casual product. There will be others as well but by product placement and by design the Series X is intended for the most discerning gamer who wants top performance in their games.
In terms of your question as to why would a PC gamer buy a Series X when they have a high end PC rig, I actually completely agree with you. I've been saying that ever since Microsoft announced that all first party titles will come to PC and Xbox simultaneously back in
2016. To me, PC+PS5 or Switch would be the best coverage for what you want to play. That said, people are still buying Xbox One S and One X. Many PC enthusiast are also just tech enthusiasts and want to get the latest and greatest tech for other areas of their home. And also, many still prefer gaming in their living room and on a dedicated console. That's what Microsoft is betting on, but I still believe that the size of that audience is smaller than the Xbox audience was at the launch of the X360 for example.