Some editions of Traveller let you determine what ship traffic is like at any particular starport. Unless you have many decent Class A starports on commercial routes, you don't always have tremendous numbers of ships in general. Now add the fact, unless they are part of the same corporation or cartel of trade ventures that happen to have scheduled trade, most ships have different destinations and time schedules and the number of ships that could convoy dwindle greatly. Ships in a convoy need extra costly computers to synchronized jump and need similar jump and maneuver capacity. That means these ships are operating at another loss with no guarantee they will need to be part of a convoy. Evoking the evil phrase 'In the real world', how many shipping companies actually send their ships in armed convoys let alone in groups? How many lease coast guards let alone military vessels to escort? As far as I know, none to zilch. The company takes a calculated chance, willing to write off the crew and try to ransom the still cheap ship back.
This is why you don't have convoys listed in the Space encounter table. They just aren't there unless a referee makes one so. And if convoys are regularly known in a system or set of systems the pirates will know too and look for real targets in more vulnerable lands. Only enemy raiders meaning to purposely disrupt commerce would be powerful and organized to hit and run just to wreck a few ships. Convoys are too expensive to be a normal sight.