If scans can early detect cancer why are we not doing them often enough?

Many reasons: First, some scans use radiation. Getting too many can also cause disease such as cancer. Secondly, it is not cost effective. There is not enough money to pay for our current health care, let alone increased screening.. These tests are extremely expensive; most would be negative, and nobody would want to foot the bill!

Very important ques.: A scan may detect benign conditions or may even suggest something is there that isn't. This leads to more tests & sometimes invasive ones. Beyond just the time & costs, there can occasionally be serious injury from the very investigation. So, there needs to be a balancing of good vs harm. Plus earlier detection must occur at a juncture where intervention changes disease course -else it's pointless.