Here’s one: Trading cards are something you own. Skins are limited to a game you’re licensing.
Here’s another: trading cards are portable; they can be put in a collection for display, put in a safety deposit box, etc. When CS goes, all the skins go with it.
Another minor one: baseball cards are informational, the skins are cosmetic only.
Mind you, I think both are forms of unregulated gambling and trading cards as well as loot boxes should have better societal scrutiny, but they aren’t identical.