The glucocorticoids released during chronic stress cause neutrophil extracellular trap (NET) formation and establish a metastasis-promoting microenvironment. Therefore, NETs could be targets for preventing metastatic recurrence in cancer patients.
https://www.sciencedirect.com/science/article/pii/S1535610824000370?via%3Dihub
Humans actually develop cancerous cells very regularly. Our immune system can fight it off more often than not, but obviously not all the time. It makes sense that being stressed would make it worse, just like being stressed makes you more likely to be sick in general.