I want to joke that the current version of the engine wouldn't allow it but I won't. I will say that I do think they purposely left it out just to make that one skill seem more useful than it really is.
Yeah, and that one skill is pretty useless anyway, by the time the Saints even show up, the enemies are already dead so the Saints just walk away. It would be nice if they would still follow the Boss even after the battle is over, but I guess we're asking too much there.
(Sarcasm) I mean, it's not like we spent a long time cycling through all the options in Crew Customization, carefully selecting our 10 favorite models out of the 40+ that's there, expecting to be able to cruise around the city with them (LIKE EVERY PREVIOUS MAIN SR GAME).
(Sarcasm) Because obviously we'd prefer to customize them then never be able to interact with them in any way whatsoever for the rest of the game... Other than a quick visit from them while we kill a few enemies...
(Sarcasm) Because that's all they are, right? Just pawns to use in battle. We don't care if they want to hang out, or go for a joy ride, we only need them to keep the enemy distracted for a few seconds while we kill them. I guess that's why they're in the "Crew" menu, and not the "Friends" menu. Because we care about our friends, but not our crew.
At least in The 3rd Street Saints everyone got to hang out with the Boss... But I guess in The Saints, The Boss is too high up to hang out with the lowly Crew.