Agree - I find that I never really have a good answer on the spot, but I often have already been thinking about the problems around the workplace for long enough that I at least have a hunch or opinion. That's not quick-wit, it's just pre-thinking. But it works well enough for me.
One skill I learned during grad school was spending lots of time going over conversations or presentations or even upcoming meetings in your head. This "warms up" your cache, and helps you play out possible Q&A, so that you have more opinions ready.
And another skill I learned was actually learning to control the meeting to a certain extent. I'd come in with something like a limited "choose your own adventure" conversation tree in my head, and then I'd try to present choices or questions to those I was meeting or talking with, so that I could at least have a fallback.
And finally with experience comes wit. The 10th time you enter a situation you're much more likely to have something to say than the 1st time. And eventually, you'll start to recognize similarities in conversations.
But yeah, lack of quick wit makes social and work situations more challenging. It's just hard to make myself have strong opinions on the spot usually.