Phones are cheap for what you get, but problems arise when you try to add something. Driving LEDs is the start. So how do you do that? Via a cloud service? How do you add an air monitor or servos? The pi and esp32s have an ecology of add-ons and a community to support you. Sure that could be built around a rooted phone, but it would take years.

You use an ESP32 or RP2040 or STM32 for that and connect it to the phone with USB. This is sometimes done with a Pi to begin with - if you need to run a 3D printer, a Pi's GPIO will not be enough. Or not, these chips might be able to handle what you need to do all on their own.