how is information making that worse? you either have a scan that shows a blip and you can use that information to inform your decision making, or you have no scan and go on without any kind of decision making. i know which option id prefer

Because you can wind up in a situation where the "blip" would have otherwise led to nothing problematic, but the followup for the "blip" actually does cause harm. And at an individual level it's impossible to tell which category you're in.

Every medical procedure after a scan has danger. Biopsies kill people. Colonospies kill people. The rate is extremely low, but test enough people who aren't at risk to have the disease and you will actively harm them.