Incorrect.
There are tons of cancers that hide and mask with symptoms common to other symptoms. Kidney cancer, for example, presents pretty similarly to both kidney stones and UTIs. Even blood in the urine isn't proof positive that anything is wrong beyond either of those conditions. And, by the time blood is in the urine, it's often too late.
Liver cancer is even worse. The first symptoms you get can be thought of as a simple pulled muscle, just a little ache in the back. By the time you have appreciable problems, like turning yellow, it's quite advanced and too late to really do much.
There are common cancers like colon, skin, breast, and prostate that more fit your description of being mostly harmless so long as you get regular screenings and eat healthy. But, for every part of the body, a cancer can form and the symptoms are very often invisible.
I'm unfortunately all too familiar with how cancer looks. My wife currently has stage 4 cancer that started as kidney cancer. She does not drink or smoke, gets enough rest, and is very active.
Annual MRI?
I do wonder if a 5 year whole body MRI or CT would be generally beneficial for the population. I don't think it needs to be Annual to have benefits.
The problem is it really isn't uncommon for your body to create random puss fill sacks all over the place. It's one thing our cancer doctor warned us about. My wife is now on a 6 month CT regimen and ultimately, they'll just ignore new lumps.
GP wasn't asking what they should personally do. They were asking how the doctor would screen for it. (The truth is, the doctor can't/won't-- an annual MRI on every otherwise healthy person, for example, would be prohibitively expensive with how MRIs are currently set up-- and as another commenter pointed out, findings from those can be just as easily ignored or put off until it's too late.)
Being beside that hypochondric point is statistically a much healthier place to be.
The current state of medicine is the current state of medicine in the actual world.
Thus, in young people cancer presents rapidly as they develop, these screenings are expensive and unnecessary. For old/sick/unhealthy people, or people who are predisposed to certain cancers, they will probably get something else anyway, so its an expensive workup to help treat a disease that won’t actually benefit much in the long term.
I’m not against treating cancer, however let’s recognize that cancer treatment is already an expensive and resource/labor intensive process. And 10yr survival rates are not great for most cancers, we’re only slowing the burn, not stopping it. Sometimes you get lucky and die of something else before the cancer can come back, but nobody is ever “cured,” they are all just delaying the inevitable. Which, as we have seen, can sometimes be worth it (who wouldn’t want another 10 years with a loved one?), but that doesn’t mean our goal should be to find a way to “cure” cancer, it should be to find a way to better manage it, and these screenings don’t seem like they really are, or at least the use-cases for them are minimal.
People talk about the “immune system” but they are really referring to a number of systems the body uses to regulate itself, more or less successfully, around environmental pressures. The body is a system under tension, sometimes extreme tension leads to extreme success (success here being growth of power), sometimes it breaks the body, and sometimes the systems have been slowly failing for a while, and most treatments will not help. Medicine is only useful in the specific case where the power of the body would be promoted if not for one thing, that the body would be healthy, at least manageably so, without that issue.