> Why does open source have to mean that you, the user, are on equal footing with where the copyright holder was before they even started? Where is that written?

It's written on the OSI page about license approval:

""" The license does not have terms that structurally put the licensor in a more favored position than any licensee. """

https://opensource.org/licenses/review-process

> Why does open source have to mean that you, the user, are on equal footing with where the copyright holder was before they even started?

Equal footing is the whole idea behind Free Software. Open Source puts accents somewhere else, but in practice it's essentially about the same thing.

> If you wanted to write a project in Rust you would have needed to be there at the beginning, too.

I can take the source code, inspect it and rewrite it line-by-line.

I can't take a closed model, inspect the data it was trained on and retrain it, even if I had enough money and resources to do so.

Whether the copyright holder will ever want to retrain it themselves is irrelevant. They have the information needed to do so if they wanted, I don't.