A simple answer to your question Kim would be because those skin designers that sell their work have earned the right to sell them, they are at the Master Skinner level. This issue has been discussed many times here at WinCustomize. I myself sell my skins, and mainly because like you, I too am on a fixed income. I do however put out free skins every so often. I can't speak for other Master Skinner's though.
I do know there are very few people designing Windowblinds skins now, as it seems the interest in them, and desktop customization in general, has over the last few years slowly become lost, sadly. [e digicons]:'([/e]
Also, please don't think that Master Skinner's make s lot of money off these sales, they don't. Reading this thread might help: https://forums.wincustomize.com/476482/page/3/#3633252