Because cancer is an absolute bastard that disguises itself as easily dismissable symptoms, that standard tests for those symptoms show up as clear. It's especially difficult if you have other ongoing health issues the bastard can hide behind.
I broke my shoulder last year and still only have around 50% of the use back. My surgeon has done everything to find out why the muscles won't work properly. I've had so many diagnositic tests and xrays, you wouldn't believe it. But they cannot find the 'irritant' which is preventing progress.
6 months ago my tiredness just got worse and worse. My GP and surgeon ordered more and more tests, MRIs, CT scans, you name it, I've had it. In the end they concluded it was just a really bad break and I need more time and psychological support because of the trauma of it all.
Then 4 weeks ago, from a regular screening mammogram, I found out I had breast cancer on that side. And suddenly it all makes sense. Should they have picked it up earlier? I mean, I've even had chest xrays, but nothing showed it.
As I said, cancer is a bastard to find sometimes, unless you're specifically looking for it.