[R0b0t1]

>Many USB drives already have small embedded CPU in them.

For most common hardware this is just an 8051 variant that sets up the USB and DMA peripherals. It's easy enough to get something more powerful, but I am doubtful you'd want to reuse consumer hardware.

[sumtechguy]

The 8051 is a decently capable CPU (it is the cpu at the heart of the furby toy). At one point they built whole computer ecosystems around it. Remember the point here is to take over the computer not have a full out modern OS. They USB manufactures use them because they work well on low power and are decently cheap and small. Now most usb sticks do not do much more than like you say. But that would not stop someone from reflashing the firmware in it who is making one of these things. The use case here is different than what most people would use it for. Sometimes you will see an older ARM design too.

[R0b0t1]

Mfrs use them because they are not patent encumbered. There are some fairly high power 8051 clones, true. But in most applications they are barely sufficient.

In this case any kind of MCU is making life harder than it needs to be.