Why do people use milk crates to build their rigs

Milk crates are pretty reasonable for open-case designs. They have big holes to allow extra airflow, but the holes are small enough where things aren't slipping through. I used modular plastic shelving for a while because I happened to have it on-hand. It worked well enough that now I use it for all my computers in my office. Bottom row is for the motherboards/etc (cable ties through screw holes so it's less likely to budge), top row for I/O & monitor. There's also the floor with high clearance under the first shelving unit where I can stuff power strips and use as general storage. Wire management is extra-simple because I don't need to try stuffing everything in one or two holes like in most conventional wood/metal set-ups. It's shockingly functional.

It's ghetto, but people are using them for mining rigs. I'm not sure why anyone would want to spend more than necessary for a functional enclosure on something which will likely be tucked away in a back room. Mining rigs produce tons of heat, and for powerful rigs, keeping them in an enclosed case often isn't an option due to airflow restriction, difficulty to access parts, and large quantities of dust building in the limited areas where input/exhaust airflow is permitted, which cause additional thermal problems. There are case designs which are pretty good about allowing maximum airflow, but they tend to be pretty expensive. Additionally, you need specific parts which fit the enclosure, usually need to purchase a few extra case fans (and they need to be exactly the right size), have limitations in the number of ports/HDDs/optical-drives able to be installed (off-the-shelf case designs are getting much better about modular front panels, though). I'm done with cases for the rest of my life, I think.