Exchange fiat withdrawal limits

So - follow on to my last post. I'm planning on putting a large amount of money into bitcoin soon. I've dabbled before, but this will be big money for me and so my scrutiny of the institutions is at a higher level. My current relationship is with Coinbase and I understand they're considered premier level as far as an exchange.

My question has to do with the nature of the fiat withdrawal limits they impose... does anyone know an actual financial / risk-management rationale for why they do that?? It just feels very sketchy to me, the rate limiting on that. Like a Ponzi scheme trying to keep up with cashflow.

When I worked as a stockbroker, if you sold your stocks they'd (back then) settle in 3 days and then your cash was yours if you wanted it. Millions sometimes - we'd wire it out. It was your money. That feels natural to me.....

So that's got me a little on edge with doing due diligence on these exchanges because they always just state their policy but it's not super clear to me why that policy exists.

Thanks :)