Is it because of elevated cosmic ray exposure due to the high altitude? Or plastic in the inflight meals? Or the crazy schedules? Or is there even a statistically significant effect over time?

Knowing any of these things is actually quite hard, and would require a lot of detailed data just to identify a potential pattern.

To add to the data confusion lol, my grandmother basically never left her home farm. Never drank. Ate meat they slaughtered themselves. Sweets were a rare treat on holidays. You could barely get her to take a car trip because it scared her. Other than her no one is aware to have ever had cancer in our families history.

Died of breast cancer.

The various US airlines, who partner with health insurance, certainly have access to this data. Cancer frequency, time in-flight, time in stratosphere, etc. Frequency of these things (or a subset) likely affects the rates.