A $39 tablet won't have the CPU power of the Pi 4, and a phone is going to have a smaller screen. Any phone or tablet with similar hardware specs is going to be comparable in price. The other issue with an app is device fragmentation--different versions of the OS, different hardware, etc. An app might work great on one phone and crash constantly on another. Siren is a great example. It works on some devices and doesn't on others, and unless you're willing to collect dozens of phones\tablets for testing, debugging device-specific compatibility issues is shooting in the dark.
With a dedicated device, I have control of the hardware, and only have to test and debug functionality on that hardware. The case I'm using has a cooling fan and the 75mm vesa mount hole patternon the back, so it can be securely mounted in a vehicle with a simple RAM mount, and stay cool enough to function even in a hot vehicle.