If it comes with the box, then that's reason enough for me.
Consider a "smart" door bell. Sure, it's easy to attach a camera to an RPi and a button, but I don't want to strap that to my front door without a nice box to put it in.
Cost will be lower with your own design because you'll integrate other components on the same board. It needs power, probably battery backup too. It needs to control some coil of electromagnetic lock. Access control is more than just flashing LEDs.
I see it as similar to a RPi - great for POC or smaller deployments, especially if you have different requirements on different projects because you can share the same knowledge base, but for a large deployment you'd probably invest the time and money into sourcing something more specific.
I've had lots of half-baked ideas for things to do around the house that this would be very cool for, since I'm not 100% sure what functionality (and therefore hardware) I'll really need.
Honestly, it's not THAT hard with Raspberry PI Pico and CircuitPython.
I have experience of programming for ARM M0 with Linaro, debugging over SWD. It was pain.
A few weeks ago I got Tiny2040, flashed with CircuitPython. I believe even a teenager can now do some microcontroller stuff. It's so unbelievably easy.
Consider a "smart" door bell. Sure, it's easy to attach a camera to an RPi and a button, but I don't want to strap that to my front door without a nice box to put it in.