Not 100% sure. But I think the proximity means that its easy for the cancer cell to spread to the lymph nodes, then they’ll get fast tracked around the body, spreading cancer everywhere. Once that happens you’re basically dead, all targeted forms of cancer treatment don’t work anymore, because the target area is basically your entire body.

You're not "basically dead" -- your odds of surviving 5 years drop from around 80% to 30%.

Again outside my area of expertise here, but to my understanding one of the significant risks to breast cancer is that if it spreads into the lymphatic tissue, it can subsequently metastasize across the across multiple other organs while compromising the lymphatic immune response that would normally help limit such a spread.