Shamelessly stolen from you know where
You can force it to happen (because the Linux kernel is quite modular and you can make it way smaller by switching off a lot of things) but as I said, what's the point when all you need is to literally run a single application that only toggles I/O ports or does a bit of comms with an external integrated circuit via I2C, Serial or SPI?
In a production design for something as simple as a dildo - so basically a motor controller - there really is no point in paying for a more powerful and more expensive microcontroller, adding cost to the final product, just to stuff a Linux kernel there to run a single application that doesn't need things like filesystem or networking support.
(For example, here is an example of about the most low end microcontroller you can use. Notice the 128 bytes of RAM and 2KB Flash storage. You can stuff enough code in there to activate a motor in one of several specific simple cycles depending on the position of a switch but not much more. Of course that kind of stuff is programmed in C either directly on top of the hardware - by literally changing microcontroller registers - or most likely on top of a manufacturer specific low level library)
Further if you do need functionalities of an OS such as multiple task/application support, there are alternatives that tend to be better for those kinds of use, such as various RTOSes.
Now, as many pointed out, there's plenty of reason to do it for the challenge or the fun of it - the ethos of any good hacker in the traditional sense of the word - just not for a production design of a device for which tens/hundreds of thousands of units will be made where adding Linux raises the needed capabilities of the microcontroller and hence the hardware cost while not actually helping to deliver the needed functionality.