There's a detail that is often missed in discussions about the use of roundup: it is not only used to kill weeds.
Roundup is also used right before harvesting as a 'drying agent' to control when the crop ripens. See, wheat is a pesky, messy living plant and it can grow and ripen at different rates even in a single field. This is an issue because if you harvest before it's ripe and dry it can rot and mold due to too much moisture. Another issue is the weather: if the wheat is supposed to ripen in 6 days but the weatherman says there'll be two weeks of rain starting in 3 days it may rot in the field before you ever had a chance to harvest.
The solution? Right before you're ready to harvest douse the grown wheat in roundup (yes wheat is resistant, just use way more roundup, it'll be fine). A few days later the whole field is nice and evenly dried out and ready to harvest. If the weather is a problem just douse it early, the yield will be a bit lower but at least the whole crop won't be lost. Two birds, one stone.
Oh and pay no mind to the elephant in the room that spraying lethal (to wheat) dose of herbicide on the kernels days before harvesting seems like a very different risk profile compared to killing weeds months before harvesting when the wheat is barely germinated. Rest assured, Monsanto's scientists have tested the flour and found that this dosing schedule makes way more money, er, I mean the amount of roundup that makes it's way into your food is way below the safe threshold.
Also I don't get the focus on cancer. There are other ailments that seem much more plausible, say, a long term low dose of a plethora of -icides wiping out your gut bacteria causing metabolic dysfunction and obesity. There has been a rise in obesity ever since this harvesting technology was introduced, but I'm sure that's just a coincidence.