Power System Design for BLDC control - why not use a DC/DC convertor?
There are a few weird things about the post so I am just going to point them all out just to be safe.

Why not just use a DC-DC convertor just before the motor unit?

I don't know what you are referring to exactly when you say &quot;motor unit&quot; but if you are referring to just the motor, then do you realize that a brushless motor driver is required to drive a brushless motor (i.e. just connecting a DC-DC converter onto a BLDC motor will lock up the motor and burn it out)?

Similarly, why not use a DC-DC convertor instead of PWM control, and have a steady voltage of the average voltage instead of a duty-cycle of voltage to get the right average?

Do you realize a brushless motor driver and DC-DC converter both used PWM?

Do you realize that a brushless motor driver is actually a DC-AC converter? optimized for high-power with less emphasis on a good quality output (since the motor isn't very picky) with features added to synchronize the output to the motor so it commutates properly?
The fact a BLDC motor driver is a form of converter means that it can, in a way, tone down the voltage for the motor.
So your question should really be why can't you just supply the BLDC  driver with a very high voltage and have it drive the motor at a reduced voltage?
There are a few reasons:

Things run less efficiently when the input and output voltage are very different (in this case, very high input voltages relative to output voltages requires very low duty cycles which don't run well or efficiently).
BLDC motor drivers are optimized for high power and the expense of output quality since the motor isn't very picky. Improving output quality would decrease efficiency which increases heat and size due to increased PWM frequency and filtering components that need to handle all the power.
It is harsher on the equipment when the input and output voltages are very different.
Without a good high quality filter (which is big and heavy if it needs to handle motor power levels) PWM doesn't actually completely hide the extra high input voltage (or the extra high peak currents that result from it). In mechanical terms, a high input voltage is like saying every time I shove something I shove it with a force of 10,000 Newtons. So to shove something more gently I apply quick, short bursts of 10,000N. That's what PWM is. You can see that it doesn't make for a very gentle or smooth ride if you don't have good shock absorbers (which is what the filter is).

This applies both PWMing to step down the power before the motor driver, and the motor driver PWMing to drive the motor at a reduced voltage level than the driver's input voltage.
So if you have an easy means of getting the input voltage close to, but above the output voltage (since it is easier to step down a voltage than it is to step up a voltage and the fundamental BLDC driver design only steps down voltage) then you might as well do it since everything will run better. For example, just swapping out the battery for a lower voltage one truly provides a smooth, clean voltage rather than crudely trying to simulate one through PWM which ends up being peaky.

Additionally, wouldn't this mean the range of operation is much higher for the motor, with no worries of running out of available voltage should we want to go higher?

Why would you want to feed the motor more voltage than it can handle? The max voltage of RC motors (where the expected operating voltages are far below the voltage rating of the insulation materials used) tends to be based on the the voltage that causes a no-load RPM that will cause the motor to throw its bearings.
Supplying the motor driver from too high a voltage could still burn out the motor even at reduced power levels, especially if the output quality is not high due to the peaky nature of the 10,000N pushing mentioned above that is representative of PWM.