Because you have to leave your studio to do so. As a producer and engineer, I can say that these speakers (and others like it, commonly referred to as grot boxes) are extremely useful. They really help you focus in your mix, on the mid range, and help balance all those mid range and mono elements in a mix (kicks, snares and vocals).
Ideally, they're not meant to be used in stereo (although many people use them that way), but in a mono, single-speaker setting. That helps in picking up stereo cancellation issues in a *poor* mix.
Lastly, the single-driver removes crossovers and the issues caused by poor crossover designs, in general. They're really invaluable tools, and when you learn to mix in them, it can speed up your mixing process and results considerably, and time is money, especially when you're doing this professionally.
So, I get that many of you hear don't see the point of these, or can't appreciate their value, but you're not the target consumer for these.
Audio engineers and producers who have these grot boxes (there are others) consider them as very strong tools in their arsenal.
Again; no one wants to be running back and forth to the car, or messing with dingles and Bluetooth speakers when they're dialing in a mix; it's counterintuitive. Also, BT speakers are more hi-fi and are pushed to try to sound as pleasing as possible, under terrible budgets and terrible designs; these are the antithesis to that. Love 'em.