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.
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.