Except the RPi is smaller, lighter, lower power and more efficient than most mini PCs. It also has a bunch of unique addon boards, many (most?) of which don't really use GPIOs, but a standard communication interface like PCIe/USB/serial/SPI/I2C.
No mini PC can beat most things about the Pi, apart from performance and compatibility, the latter of which is fully the fault of the manufacturer.
An arduino board with a esp32 added to it can do a lot of things(minus some cam/image recognition stuff) for half the price.
Rpi is strictly suited to very specific use cases.
I've deployed my fair share of both and SBCs are still a very widely useful middle ground. Being able to use existing Linux tools instead of limited or nonexistent embedded libraries is a huge benefit. Most things to do with sound or video, for example, are totally infeasible with microcontrollers. A mini PC is noticably bigger and you need to add a microcontroller over USB if you need absolutely any real world interaction (even something as simple as sensing ambient light to adjust brightness).
My new favourite tool for these situations is the Radxa X4, which is the exact dimensions of a RPi, but with an Intel x86 CPU and an onboard microcontroller to drive GPIOs.
Yeah rpi does have some real world usage but for many small projects that hobbyists do rpi can be a overkill.
Will checkout the radxa x4 sounds interesting.