Why are switching frequencies for boost converters above the 100kHz
A powerful boost converter could operate in the low/medium kHz range and might do so because the power transistors used are inherently slow devices. The trick is to operate at a frequency where static losses approximately equal dynamic losses.
If I understand correctly, as the frequency increases from 100kHz
upwards, the ripple current that is created from the inductor
decreases, the current change over time decreases in the inductor, and
components can be smaller because they don't have to deal with larger
Ripple current sets the scene for how much energy is stored by the inductor and given to the capacitor cyclically. At higher frequencies this transfer is done more times per second hence, for the same power delivered to a load, the ripple current could be smaller but this doesn't quite deliver the same power (energy proportional to current squared) and so the inductance has to be reduced and this increases the ripple current. If you try and factor in the possibility of running discontinuous or continuous conduction mode then it's not as clear cut as you might think.
Components can be smaller, yes.
However, they're countered by decreased efficiency from switching
losses in the MOSFET, as well as losses from the core of the inductor.
Yes and no. Switching losses do increase but some core losses reduce such as saturation. However, eddy current losses (usually smaller than core saturation) will tend to increase and that is why you see significant development in making cores suitable for switching above 1 MHz.
So, given that you can increase efficiency by decreasing frequency,
why don't switching frequencies occur in lower ranges; the 100Hz-10kHz
range, for example?
At low frequencies the inductor saturation is a big factor - lower the frequency and saturation losses can suddenly sky-rocket. If you maintain the balance between dynamic and static losses in your MOSFETs that is usually the best frequency to aim for (as mentioned early on).
Is it that the current changes that the inductor has to deal with are
too high and the inductor wiring resistive losses starts to dominate
as the main source of power loss?
Lower frequency means less energy transferred per second and this means you have to run at higher currents (for the same power out) but don't get obsessed about this. Running CCM (continuous conduction mode) means the ripple current can be very small to transfer the same energy.