Because for $40 I have a system that runs at a decent speed.
For $300 I could get an ITX to run.
So for the cost of an ITX, I could run a dozen RPIs. Who wants to have a server running in their bedroom? Have you heard the noise those things make? Sorry, no.
A “server” doesn’t need to mean a pizza box with 15k rpm jet engine fans.
My server is repurposed desktop hardware in a desktop tower case and is nearly silent except for the subtle hard drive noises. The hardware cost next to nothing and is far faster and more capable than any pi (except the pio of course which wouldn’t be used anyway).
You’re running the pi and drives in a plastic take away container off usb power for that price.
At the very least you want the case and psu. At which point the question is which cpu+motherboard+ram combo do you want in that case. The rpi is one of many such options and is actually quite expensive for the amount of cpu+ram you get for the price.
An ITX isn’t the competitor for a Pi. I’d suggest a USFF prebuilt. I use an HP Elitedesk and Dell and Lenovo each have similar tiny PCs. They’re nearly silent or completely silent, and half the size of a Mac Mini. Cost is about $150 for hardware that is more than enough for me, plus they can have 1-2 SSDs and a hard drive inside the case.
Clarification: They're about half the height of the OLD Mac Mini. Better comparison: They're the size of a typical hardcover book if you chopped it to be square.
I'm uncertain of why $40 vs $300 is even a point of debate on HN. The latter is a one-time investment and you likely can expand it a bit i.e. add a 2.5" or M.2 drive later.
What's the gain of running 12 RPi, exactly? Do you do research work requiring distributed low-cost computing?
They are if the GPU can be attached. I avoid virtual machines in favor of container workloads from containerd for this reason. It’s easier to attach Mali GPU and do my work than it is to find cash in this economy for a dozen RTX’s.
For $300 I could get an ITX to run.
So for the cost of an ITX, I could run a dozen RPIs. Who wants to have a server running in their bedroom? Have you heard the noise those things make? Sorry, no.