When it comes to something like this, the quality of the software makes all the difference. And good software development does not come cheap. I guess with products like this you are probably paying for the software as much or more than the hardware. Hardware and assembly costs can be minimized to virtually nothing, and we'll still need to pay for decent software if the overall product is to useful.
It may be that for some reason PC software development costs more than embedded code. Or, a physical user interface eliminates the need for a software interface and all the features we get from a standalone device.
So, perhaps I've answered my own question - we may see inexpensive USB scopes, but I guess we won't see anything really worthwhile until we step up and pay for the software that allows that to work. Those Hantek USB scopes might be closer to Pico than we realize, but the software makes Pico all that much better (kidding a bit - Pico is no doubt much better designed than Hantek)