The A in SS7 stands for authentication. It's old enough that it predates any kind of meaningful network security practices. You can amend the standard to add auth (that's what SHAKEN does), but the issue has been that much of the equipment carrying these calls was built back when C was a hot new language and people rode dinosaurs to work. Some telcos based their entire business model on putting off the upgrades and none of the legitimate telcos would cut them from the network, so the FCC (eventually) stepped in.

I know what it's used for. I just don't care. Corporations can find another way.

It's to allow for trunked calls. If you're calling out from a call center, it allows the caller ID to reflect the public phone # that people can actually call rather than the internal phone # that was used to call out.

It's a legitimate use, but comes with a high potential (and reality) of abuse.