The "why not to start your own store with wordpress" here would be that you are "NO ONE" having your own site, you have to drive traffic, is just way too difficult to get traffic and then CONVERT that traffic? Even tougher.
You have to go through a lot of paid ads on FB, IG, etc to try to get that people to go to your site and if you dont know your demographic good enough, then the people going to your site wont buy.
So the "easy solution to that" would be just use Amazon/eBay platofrm to get said traffic, the problem would be that you have to pay 10% fees (give or take) to them for having "the honor of selling in ther site"
So yeah, not saying its not a good idea, more difficult and more investment to try to get traction