Why does the fan speed up like they do when there's a firmware update?
The fan speedup happens because normally the fan speeds are controlled dynamically in response to CPU temperature by the firmware (and as the CPUs are typically underutilized, there is rarely or never any need to run the fans at anything like the maximum speed).

However, when the firmware is being updated, that dynamic temperature control is not possible - for safety the speed is just set to the maximum and left there until the update is complete.

This is true for all systems (even PCs - you will often notice a high fan speed for a moment when booting them as well)

So the real question is why is the difference between normal and maximum fan speeds is so great in the laptop (MacBook) systems as opposed to Mac desktop/tower/mini systems - and the answer to that is simply that the laptop hardware has much smaller clearances and therefore poor airflow without fans running.  So for safety and increased CPU speeds on laptop systems, more powerful (louder, faster) fans are needed (although they will eat your battery, as much or more than the increased CPU power consumption itself when the CPUs are fully utilized).