For the same reason that the price of bread, peanut butter and ground beef rise and fall together regardless of the grocery store you use and are are priced similarly in a rejoin. If the demand for bread rose the grocery stores would all raise the price of bread at approximately the same time. No collusion that’s just how it works
The market keeps the price as low as the demand will allow.
Everything else, or almost everything else you buy behaves this way across brands.
Do you not sense a significant difference between approximate timing of price rises in bread and milk, and simultaneous rises at gas pumps?
Am I overly sensitive in this regard?