great question. so many possibilities.
What the name of that thing that in a lot of industrial buildings, offices, schools, etc. they have instead of ceilings these cardboard-type tiles that look really gross and depressing and brake down and get moldy? Maybe there's some reason for them to exist, but I'm going to say those things. Or maybe fluorescent lights which tend to go together with them.
The main reason is "drop ceilings are cheaper to install, demo and maintain than the alternatives" secondarily they give a place to put sh**t that you need to get to without it making the room look messy. In short, cheapness and ease of maintenance for pipes and wires and such. If you're not doing maintenance or paying for it, I guess you can bitch about the aesthetics, but the people paying for it and working on it aren't going to listen very long unless you've got something better in mind.
Sameish story with the lights. Cheaper, by far, than the alternative (at the time), easier on the environment if you care about such things, and much easier on, again, the maintenance team who had to go around replacing bulbs all the time. Florescents last much longer.