Ships nearby responding to a distress call is a matter of expedience rather than good planning - something is better than nothing. But typically once an organization like a coast guard gets involved, they start giving orders to other responding ships, including sending ships away if they aren't needed and adding to the fray. This general concept is called incident command or the incident command system (ICS) after a set of practices that I think originated in firefighting but are now broadly taught by FEMA to all sorts of disaster responders. Basically that there needs to be someone in charge of the incident and there need to be standardized and controlled flows of information, otherwise it's very easy for the response to be ineffective and even dangerous because of poor communications, miscoordination, etc.

There’s a bit of a difference between a ship already at sea altering it’s course to get near and assist a vessel in distress (where they might be first on he scene), vs ship going to sea especially for an event I guess? You don’t want the area too crowded with “good Samaritans” who’ve all gone to sea just to assist.

I recall some comments saying that operators of a submersible that could hypothetically rescue a sub stranded on the bottom being discouraged from deploying for the Titan by the coast guard, which perhaps means they already knew the fate of the vessel. Although it could be that they already had a suitable submersible arranged already and didn’t want more in the area causing complications with coordination etc.