Why do you need other people to do a rally and not have an option for solo?

I feel the need to specify because I didn't think clearly when writing the question:

I'm regarding solo rally when completing missions as I tried earlier and it cancelled saying "Not enough players".

I understand teamwork but really? It's not like the guild I'm in has active players.

P. S.: I choose side quests first before fighting so changing guilds is a big no no for me.