Wood burns, plastic melts. When you have a bunch of racks full of a bunch of servers undrler heavy load, managing the environmental temperature of the datacenter is very important.
One of the jobs of the case is to contain a fire if it starts. A plastic case wouldn't do this, and a wood case would actively be hazardous.
1) Several appliance machines were in a cabinet with water-cooling doors attached. The doors restrict the airflow from the front of the cabinet to the back to the point where there isn't sufficient airflow to cart away the heat from the CPUs. The engineer responsible for those particular systems played show-and-tell with the melted plastic pins that formerly held the motherboard in-place for a month after the machines crashed.
2) An 8-year old rackmount machine had a power supply fail spectacularly and light the entire server on fire. Fire suppression was triggered in response. Several other adjacent machines were damaged but the fire stayed relatively contained to the one cabinet.
Yes, but no. Yes, that will work. No, that will very likely not be compliant with your local RFI regulations.
That being said there is little excuse for cases with non-deburred edges when you can get cases that are fully painted (in and out) and have most cut-outs folded over for surprisingly little money (~50 €, which really makes you question how any of the questions make a profit off of it, not to speak of workers involved).
1) Even water-cooled equipment uses forced air cooling to cool the water heat exchanger (unless you happen to own water rights to a river, in which case we have a different discussion). A lot of the highest mark-up goes to bespoke on-prem data centers (e.g. hospitals, labs) where the air isn't necessarily cleaned to spec. I would therefore think you want some stuff between the air and the CPU heat exchanger to force the air around some turns to knock some of the debris out of the air. So a grill makes sense.
2) Most buyers are buying single units, but most units are sold to buyers buying them in bulk, with a service contract.
3) If you're making a grill, it's gotta look like something.
4) You want something that approaches a consistent theme for your accessories: monitors, keyboards, mice, printers, etc. Even without the logos, you can match the HP mouse to the HP keyboard and HP server. Same for Dell, Lenovo, etc.
5) I find the big manufacturers try to slowly evolve their design over the years. You can walk into a place (hospital, power plant, ship, whatever) and quickly identify it as a Dell shop or an HP shop.
6) Electric discharge machining makes it relatively easy to go from CAD drawing to ready-for-production molds.
7) You need some low-risk projects for your young mechanical engineers.
8) At scale, your custom EDM-mold plastic grill may be cheaper than stock filters of sufficiently similar performance.
8) Branding matters.
This is one of the purposes of a Design Language -- it ensures users of your products can identify what does and doesn't go with your products. And it also ensures that if a user has used one of your products, the signifiers for your other products are immediately apparent to them.