I think the reality is that a lot of people don’t use or care for the main Pi features like the GPIO pins or camera connectors, and aren’t using them in places where space or power is constrained. They’re purely looking for a cheap amount of compute.
For those use cases, it makes sense that performance per dollar is the top metric that many people look for. Pi used to be king there too, but as Pi prices have gone up and the price of other cheap compute has come down, it’s harder to justify the Pi. The Pi is still a great board, but a Pi5 with case, power supply, SD card will run you $100+ (throw in an SSD and you’re at $200+ easy), meanwhile an N100 PC can be had for the same price.
It's kind of like a keyboard without LED lights for status, pop up tabs for the angle, and a recessed guide for the USB cable. Would it technically be cheaper without those things? Sure... but it also costs next to nothing to add them. Once a keyboard gets to $10 almost all of the cost is already put into "being the best keyboard for $10".
There are certainly cheaper, <$100 TCO, alternatives to the Pi or N100 PCs... but they are accordingly worse performing. One thing Pi is really good at is having well supported options on the ultra low end. The Pi Zero 2W for $15 - it's crap performance but cheap. The Pi Pico microcontroller starts at $4 if you don't need a full OS (or just need to augment an existing box with GPIO over USB). If you're building a PC out of a Pi it's just not a differentiated option vs an actual PC is all.
I think software compatibility and power supply are two major issues. Non-x86 boards generally only run manufacturer supplied outdated Kernel, and x86 units are often only compatible with included AC supply.
Devices that solves both of those problems tend not to be price competitive with Pi, and many ends up paying for a Pi.
For those use cases, it makes sense that performance per dollar is the top metric that many people look for. Pi used to be king there too, but as Pi prices have gone up and the price of other cheap compute has come down, it’s harder to justify the Pi. The Pi is still a great board, but a Pi5 with case, power supply, SD card will run you $100+ (throw in an SSD and you’re at $200+ easy), meanwhile an N100 PC can be had for the same price.