Unfortunately "cancer" is a very broad brush that covers a multitude of diseases.

Plus the phrase "cure" does a lot of heavy lifting. People seem to see a win here as being "here's a tablet, all cancer is gone."

So yes, we have spent an insane amount of money that can be ascribed to "cancer". (We've Also spent a lot on heart disease, diabetes and so on.)

But yes, we have got an extraordinary return on money spent. Treatments and survivability of common cancers (breast, prostate etc) have gone through the roof. Better screening, better education and much better Treatments lead to much (much) better outcomes.

Not all cancers are the same though. Some are harder to treat. Some rare ones are hard to investigate (simply because the pool is too small) but even rare cancers get spill-over benefits from common ones.

In terms of "cure" - that's not a word medicals use a lot anyway. Generally speaking we "manage" medical conditions, not cure them. "Remission" is a preferred word to an absence of the disease, not "cure".

In truth, we all die of something. Cancer is usually (not always) correlated with age, and living longer gives more opportunities to get cancer in the first place. So it's not like we can eradicate it like polio.

Imagine if those trillions would be spent on research and healthcare

That's on the same level as terraforming Mars to escape climate problems on Earth.

Removing cancer from a body is tremendously simpler than making a new body.

The brain cannot function outside the body. The brain needs your bone marrow to make red and white blood cells. The kidneys and the liver to filter and break down metabolic waste. Various other hormonal systems that affect how the brain works (c.f. the HN favorite "gut-brain axis"). A brain separated from the body could survive for a few weeks, but long term it would certainly die from neuron loss (i.e. dementia).