> I wonder why?

Yes, I also wonder why you'd take that to mean it caused cancer.

The thing you quoted doesn't indicate anything about it causing cancer. It says people with a certain medical condition shouldn't take it. Hormones can influence how fast certain cells get replicated, and it's important to not encourage cancer cells to replicate even more.

It's because cancer treatment fucks you up, big time, permanently, and a lot of your healthcare thereafter is going to need to be changed to accomodate it.

This is a big part of it. Most cancer treatment is literally "we're going to get as absolutely close to killing you as we can, and then stop at the last second, over and over and over again, and hopefully get you into remission."

You can't do that and then turn around take drugs that mess with your hormones and expect everything to be totally fine without even a single conversation with a physician.