why cant i buy games on my windows 10 pc through xbox?

It's because of whatever deals the developers have made with the platforms. So in the case of Modern Warfare, there's just no deal to sell the game through the Microsoft Store for PC. Since it's not available for PC through Microsoft, it can't be a Play Anywhere title.

You shouldn't have to buy it twice. The game is an Xbox Play Anywhere game, so if you purchase the dlc from Windows 10, you should be able to download it to the console at no extra cost. Check to make sure you are using the same account on both
the Xbox One and Windows 10. If you are not, then that will be why you are being asked to pay for it again.

This did not help I still cant not update the ARK version on my computer I can update on my XBOX but not my computer and I cant figure out why and I am using the same account because I updated my xbox by my computer.

windows 10 minecraft cant be played on the console. as thats a pc version. you would need to buy the bedrock edition (i believe)

When you buy an Xbox Play Anywhere digital game through the Xbox Store or the Windows Store, it’s yours to play on both Xbox One and Windows 10 PC at no additional cost. To take advantage of Xbox Play Anywhere, you
will need to have installed the Windows 10 Anniversary Edition update on your PC, as well as the latest update on your Xbox One console. Then, simply log into your Xbox Live/Microsoft account and your Xbox Play Anywhere games will be available to download.
On your Xbox One, games will show as “Ready to Install” under “My Games and Apps”. On your Windows 10 PC, games will appear in “My Library.”

Participating games will be marked on www.xbox.com, in the Xbox Store, and in the Windows Store with the Xbox Play Anywhere icon. You can also go to www.xboxplayanywhere.com to see a full list of participating games.

I m trying to record the game while playing but

this error occurs "we cant record now please try later"

is there I have to buy x box for windows 10 pc separetely