As others have said, its about the false negatives, causing harm.

Additionally, screening Methods themselves arent harmless, sometimes. I dunno the specifics of pancreatic cancer screening, but, eg., the colonoscopy checkup every 10 years is invasive in itself. While you provide a (proven) benefit to every 6 of 1(00?).000 patients, you also cause severe complications (bleeding, death) to 3 of 1(00).000. While in this case, you have a net sum of 3 people being helped (prolly being more since you could manage some complications, I think), it may not be the case for rarer cancers.

If for fishing out 1 out of 100k [obscure cancer] you have to hurt 2-3 out of 100k, its just not worth it. Not to mention since screenings usually are not gold proofs, you need to do actual diagnostics, which can be hurtful in terms of (minimally-)invasive procedures, high radiation exposure and hospitalization stays.

We have to wait until we get something that allows us to diagnose a patient with 100% accuracy while having no drawback for those rarer cancers, sadly.