It's more the opposite. The hardware level updaters, are usually small pieces of non-updateable code called bootloaders. These are purposefully made small, slow, and ROBUST. They do parity checks on everything, status checks, store local copies of data transfer in weird esoteric ways to not be disturbed during the actual update.

The issue is that you can make it work 99% of the time without issue way faster. But bricking a car that then has to be towed for free to a service center and flashed is super expensive. So the fix is slow and robust updates that happen when the user isn't even there.