But why is that?

When you invest in the development of the game, and you release it on Steam. Whatever you spent on the game's development is not an investment in Steam.

When you start a Wordpress site, and pay for hosting and the domain that is not an investment in Wordpress.

Because saying otherwise would be special pleading, as established by these examples that is in the post. So there is more to my argument than because I said so.

If you have an actual argument on why you disagree, I would be happy to hear it.