Why might there be a risk of cancer recurring even when surgery is performed to remove a malignant tumour?

Microscopic residual: Depending on what type/stage of tumor (possibly some other predicting features as well) some tumors may leave behind microscopic cells which are undetectable by current medical testing (bloodwork or imaging). Even just one of these cells may grow back into a tumor and therefore additional treatment (chemotherapy/radiation)after surgery may be needed to help 'clean up' residual microscopic cells.

Here are some ...: Reality confirms no pre-treatment imaging studies such as bone scan, CT, MRI, etc, can offer 100% accuracy. That is why many patients may show recurrence at local, near, or far sites after definitive Rx intended for "cure" such as radical surgery, radiation, etc. due to their inherent lack of sensitivity & specificity to detect "microscopic" spread, So, such risk is always there, but just keep ..

Preoperative risk : In most cancers, we can determine risk of relapse or risk of death based on stage of the cancer I.e. Stage 1 > 2>3 etc. The risk also depends on the biology of the cancer cells - I.e how aggressive they are. During surgery , visible cancer is taken out. But microscopic clusters of cells may remain in that area or some may have gone via blood to other parts. These over time cause cancer relapse.