In a word, TANSTAAFL. This is nothing I have not thought of and in a few words, it isn't quite as straight forward as you make it out. The Melangers use a relatively inexpensive but durable capacitor start motor. If you drop the speed on a capacitor start motor with a router controller they can stop running as the rpm drops or a high load is placed on them. Basically, under the conditions used, it would stall. To circumvent that, yes, they could use a non-capacitor start motor. The result would be a much more expensive motor and an inexpensive controller - with no net gain at all in price.
If you want to dig in, check these out:
router-speed-control-how-does-work
Will-a-variable-speed-control-meant-for-routers-work-on-a-grinder
So why don't they just use a router motor (you may not have asked that, but others have...)? It's only $50.
It's an rpm issue. The Melanger runs at about 1200 rpm and is geared down with a pulley to about 100 rpm. Routers can and do run at twenty times that speed. By the time it reaches 1200 (or even 3000 rpm) the torque plummets (the router controls are only linear to about 20-25% power) and effectively is useless (or will burn out with extended running). And if you keep it higher, then you need more gear reductions again, and once again, no net gain (reduction) in the price, but you have added complexity and many more failure points.
To repeat - TANSTAAFL.