How does one even accidentally steal a texture someone else made?

The fact that you're asking this is almost like...maybe you don't know what you're talking about? And should defer to an actual creative who does?

First off, it is a famously non trivial problem to compare every texture to every piece of art on the internet. It is trivial to add a bit of impercievable noise to deliberately foil even the best reverse image searching methods.

I think you may be taking for granted the number of artists and the level of autonomy they are given over their craft for a project of this complexity.

It's actually more weird to me that you don't understand why this is easy for a single dev to get away with. This is what happens when a studio trusts its artists to create something. No one is excusing it, but stop acting like Bungie did it on purpose. There's no evidence of that. The only thing they're guilty of is making a mediocre extraction shooter.