I'm thinking we need them yearly.
As good as it sounds, this will never happen. At least not anytime soon.
MRI is still fairly expensive, and doing full body scans will drive up health care costs to unbelievable levels. No one would be able to afford health care.
In addition, it can't tell you if it's cancer or not. Incidentalomas are extremely common and majority are benign. Do you want surgery on things that are benign? Think about this from the doctors perspective - First, do no harm. Even if you didn't do surgery, and you just wanted to try and diagnose it with a biopsy, there are still risks.
This is why medicine has cancer screening tests for individual organ systems. Yearly mammograms/ultrasound for women > 30. Colonoscopy every 10 years for everyone > 50. Pap smears every 3/5 years for women. We are likely going to start doing CTs for lung cancer (most common cancer by far). People with increased risk will get checked sooner and more often.
Not all screening tests are perfect (PSA) nor do we have any for some organ systems (renal, endometrial, ovarian). But it's getting better. Cervical cancer has plummeted in the era of the pap smear. Cancer sucks. But there's hope.