It’s all about math and the almighty dollar impact to the bottom line.
They did a calculation of the MTBF of the pump, the expected lifespan of the vehicle, the number of pumps that would fail in that lifespan under warranty, the number of vehicles expected to be sold, the cost of the access panel and the additional cost of service without the access panel.
It was cheaper to the corporation to pay the few extra service dollars to drop a tank on a small handful of warranty repairs than it was to add the access panel to every vehicle.
Non-warranty repairs never factored into the equation, since that becomes either dealer service department profit or “not my problem”.