I'm not an expert here, but yes, there are a lot of incidents and accidents that need to be investigated that don't involve the ship sinking (remember the Ever Given?). Plus, having a device installed in some kind of floating pod which detaches automatically when the ship sinks requires a lot of extra equipment, it has to be on the outside, so exposed to salt water and corrosion, etc.

Most ships sink in relatively shallow water. Broadcasters may fail, especially in the conditions that sink ships. In that case, finding a ship is easier than finding a floating box. It also simplifies the design a lot.

I believe all ships have a transponder.

Russian criminals were stealing rich boys million dollar yachts a few years ago.

They would board the ships when the skeleton crew was off the ship.

They would take the transponder, and put in model hobby ships.

The owner, and authorities thought their yacht was exactly where they left it.

The Russian thieves would take the yachts back to Russia.

(I thought is was clever. They stole millions of dollars in terms of yachts, and were eventually caught buy Interpol.