The reason isn't really security, it is mostly to hide the wire monster from my wife's delicate sensibilities and to further drown out the fan noise. One of my closets had an AC duct, which I basically enclosed in a little room to have a "consumer" grade server room (I close the duct in the winter, my temps are fine). The little room has a "crawlspace" panel in the plain old drywall wall, which is pretty low-key and not at all hard to crawl through for the rare times I need physical access. It cost me about 100 bucks at home depot to buy a handful of studs, a sheet of drywall, and a crawlspace panel. I added smoke alarms on either side, because I am completely aware that this isn't the most fireproof of solutions.
Honestly it all started with me putting a 16 port switch in the attic, then realizing it needed to be in the A/C, then moving it to the closet, then my very patient wife (bless her) casually suggesting that closet was becoming a cesspool of discarded hardware, wires that went to nowhere, and loud weird equipment..
A side-effect of this is that the average burglar would barely even be able to tell that I spend a small fortune in very geeky and completely unnecessary server hardware.