Why pool of processes?

I would imagine that it means a collection here. This from OALD - a supply of things or money that is shared by a group of people and can be used when needed

For example:

A car pool

Let's pool our resources.

We have a pool of free-lancers we can call in if necessary.

RandomIdeaEnglish has the right idea. The origin is from this definition of pool:

any communal combination of resources, funds, etc: a typing pool

While you're correct that collection/bunch/group would make sense in reference to processes, the term dates back much earlier in reference to other types of shared resources. I believe memory was the first, though I can't find confirmation of that, only this citation from StackOverflow:

The term heap (meaning memory pool) was used at least as early as 1971 by Wijngaarden in discussions of Algol.

It would not make sense to refer to a "bunch" of memory, or a "group" of memory. Nor would it make much sense to refer to a "heap" of processes. Whereas "pool" works equally well for a wide variety of resource types.

According to the Online Etymology Dictionary, the use of pool in the sense of "common reservoir of resources" dates from 1917.

Other terms like list or set don't embody the sense of a common reservoir, which is explicit in this definition of pool.