Why was the IBM PC keyboard so loud?

They didn't design it to be loud, they designed it to be reliable, less expensive to manufacture and to have excellent typing feel. The basic design was patented in 1971 and later variant in 1977.

It's important for touch typing to have a good hysteresis between make and break and to have a uniform and progressive pressure building up to break-point. Some sound is also helpful.

Far from being loud, it would have been quieter than a room full of typewriters, let alone line printers.

https://sharktastica.co.uk/wiki?id=ibmbucklingspring

https://patents.google.com/patent/US3699296A/en

https://patents.google.com/patent/US4118611A/en

All that being said, I use a keyboard with cherry green switches and they're OK. It's not got quite as nice of a hysteresis curve, but you can see from the diagrams in the links above why that buckling spring is just a really clever way to make a very nice feel.

They were marketing them to parents with children who had a tendency to sneak onto the computer at midnight to play LORD when the BBSs reset and everyone got a new turn. Lost my computer privileges more than once due to those clicky keys.

It was a derivative of their prior knowledge and experience making the IBM selectric golfball typewriter. Different tech but grounding experience.

The B in IBM dates from them selling cash registers and calculators to business. They'd been in keyboards a long long time.

Because to keep people comfortable, and make them adopt it, it needed to have some characteristics in common with typewriters. I think.

It may have been designed to emulate the feel and sound of the earlier IBM Selectric typewriter that businesses had been using.