The ships apparently do not follow set routes. I imagine that insurance companies could make routes more uniform, but ships do not all travel between the same ports. If convoys were useful, I think they would use convoys. Apparently they are not a complete solution to the problem.
I do know that the naval authorities say that the reason for piracy is that the area is to vast to patrol it all. I think that they are better experts than, say, sirs or kramer.
I don't have a degree in it , but I was a sailor and I have sailed in that area, I have made port o'call in Sudan, Kenya and have transited the Suez.
Early in WWII there were a lot of losses to U-Boats and convoy tactics reduced the losses, there was resistance from ship owners and captains because convoys are not easy to manage or particapate in.
I think as long as the pirates are attacking from small skiffs they will be quite vunerable to the small arms that can easily be mounted on any ship and helpless to fight any real warship.
You may have a point that it takes a huge and impractical fleet to patroll the entire Indian Ocean , but my counter point is that it would only take a few warships to escort a convoy even if the convoy was quite large.
The inconvienience of forming convoys would be made up for each time that a piracy was foiled.