I own original Sims 1 & Sims 2, why do I have to buy it again?

You're paying for the work that it took them to adapt these games to run on Windows 10 and 11. If you don't want to pay because you aleady own older versions, you don't have to. But if you can no longer play the older versions on your computer that answers your question. 

That said, there are so many issues with these games that I would not buy them until and unless they fix them.