Devkits aren't monolithic. Particularly before a console is released, they go through many iterations which look wildly different. Example, someone above posted
A PS2 dev kit. That's not the only one. For example, here is the pre-release PS2 devkit:
Pre-release Dev Hardware is usually radically different than the release hardware, specifically because the specs are up in the air and also because they need room for the equipment to run the overhead. For example, Sega Katana devkits feature a motherboard much bigger than the stock Dreamcast motherboard because they populate much more additional RAM, which allows them to hold things like the debugger in memory while still using the rest of the system. This means the that the early case usually looks very different.
Regarding "generic, PC cases" -- this usually is NOT true of pre-release dev kits. They might look like actual boxes, yeah, but they are almost never just a generic PC case. They are almost always some sort of bespoke box, precisely because the hardware isn't fabricated with form factor in mind. This doesn't mean they are always ugly boxes of exposed wires, either. Depending on the company, they might use brushed steel or polished designs even for their unfeasible devkit box. Examples of early Xbox dev kits:
VS the debug kit they sent out to commercial devs post-launch:
Which actually became the basis for the "Xbox Halo 2" model.