system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is heart cancer so rare?

The heart's an organ just like the brain, right?
gpt
This is because muscle cells (also called muscle fibers) don't divide, and cancer is most common in cells that divide a lot. Dividing cells are primed for cancer because:
1) a lot of the machinery needed to divide rapidly is already present in the cell so not a lot extra is needed to make it grow uncontrollably into a tumor/cancer
2) the more a cell divides the more chances it has to make errors. DNA replication isn't perfect, and over time these errors can accumulate into dangerous mutations (this is also why cancer is more common in the elderly)
This is why we see a lot of cancer in tissues with a high "cellular turnover" rate - skin, liver, colon, etc. Cells that never divide (muscle fibers, neurons) almost never get cancer. Brain cancer is usually caused by a special type of brain cell that can divide (but not signal) call neuroglia, which forms the cancerous "glioma."
Bonus biology fact - because muscle cells fibers don't divide, you have the same number your whole life. Arnold Schwarzenegger has the same number of muscle fibers now as he did as a baby, they're just more packed with the good stuff.
Source - biochemistry major, medical researcher.