IMHO wide-area merc pools kinda make sense, as warlords looking for soldiers for hire AFAIK tended to send out all kinds of agents and intermediaries to find sellswords - and conversely the mercenaries themselves, especially those already in the business, would keep an eye out for work opportunities over fairly wide areas. Converging interests, as it were, and both presumably generally had a decent idea of where and how to find the other.