Often, especially at the beginning of the season, the outward manoeuvre is not done under the best conditions. Processes have to be reorganised and the sailors are not yet back in sailing mode. In addition, the stress of the working week often lingers on during the weekend manoeuvre. Once back on the boat, it's time to cast off and get underway.
On charter trips, there is also the fact that the ship and area may not be well known. And given the limited time available, there is a growing temptation to set off even in sub-optimal conditions.
However, various examples of crews having to turn back or at least experiencing a nasty surprise in or shortly after leaving the harbour show just how important it is to pay attention when casting off and leaving the harbour.