Beyond the fact that the most common sensors/peripherals are built in...I can find a lot of USB sensors that work with Android. In general you can also drop down to NDK (c code) on Android if you want.
So I'm still curious on which sensors would not work with a Android phone that people would want to.
Let's see... USB sensors add unneeded complexity AND power requirements.
How would you stick an Android phone plus a whole octopus of usb peripherals inside a nice neat box? It would be at least shoe box size.
If you need a board that will run 24/7 and control some process, remember Android goes into power management to save battery, and you need to fight it to operate continously. Might not even be possible any more with newer Androids that restrict wake locks.
RPi reliability isn't as good as it could be, at least not without cooling, but I would trust it for long time operation any time over a $30 phone that had to include a screen and a battery in that price and thus has made a lot more compromises.
Literally tens of thousands of them. Sensors are not interchangeable, if you have requirements on precision/accuracy for your project then it dictates what you have to use. And often enough the on board in a phone sensors aren't reliable (e.g. temperature sensor on a phone is useless for measuring ambient temperature) or not precise enough. Or other things like driving a stepper motor using an SPI-based driver, controlling a character display or interfacing with an external DAC/ADC.
It's important to note that I'm talking good old electronics projects, not things like single-use kiosk devices. Like I said earlier, the RasPi is similar to the Arduino in that it's suited to "edge" tasks - interfacing a computer / network with the physical world through sensors and actuators.
Some of this stuff is possible on an Android device, but more complex because they aren't designed for it. Meanwhile this is what the Raspberry Pi was always intended for and is much simpler. There are cases where an old phone might work better, but there are many where it won't work at all or would be a lot of extra work.
USB sensors are more expensive and need an USB hub (which raises the price, what's the point then?).
Not to mention most sensors (humidity or CO2 for example, or even just a simple potentiometer) are not USB (and for a good reason, I don't want/need USB chips and connectors in each one of my sensors).
Example applications I use Pi for: automated irrigation, custom digital musical instruments.
Beyond the fact that the most common sensors/peripherals are built in...I can find a lot of USB sensors that work with Android. In general you can also drop down to NDK (c code) on Android if you want.
So I'm still curious on which sensors would not work with a Android phone that people would want to.