Because it’s not legal and no one’s going to develop software for XP. Someone could make and sell security patches for it, but the type of person who still runs XP either doesn’t care enough to buy security patches or it’s running some hardware that isn’t connected to the internet.

There are exactly two games released in the past few years that have XP support, but that was more a flex on the part of the developer then catering to the market. HROT and Zortch are those games if you’re curious.