Why is Rising Tide solo only?

I already have a railjack, but my swarmmate needs one to do the new war, and is having trouble with the missions. I had trouble too when I got my railjack. The enemies were easy to kill, but on the earth mission in particular I struggled so much to keep enemies out of the area, since it had two sides with a big rock in the middle. I couldn't cover both sides at the same time so it took ages. I saw a lot of people complaining that each player is allowed their own railjack, but I have no problem with that. My complaint is why can't you help your friends on the quest? There's no story reason for it, it seem silly.