Diablo III, Why people always do the same in Adventure mode?

I've played around 70 times on Adventure mode (in Normal difficulty, and played other modes) and people always do the same, no matter the difficulty mode (unless Tormento), they just does the bountys of Tristam and after that all the people leaves from game (maybe to join again other party to repeat the same procedure), I've seen people with +300 paragon levels doing this all the time, seems not reasonable to me 'cause they could be playing on Tormento mode to drop more and gain more exp, exist a reasonable reason to determine this behavior of almost all the players?

Of course sometimes (very rare sometimes) a group can do other bounties of other maps but from my experience is very strange to see it.

I don't judge other players, I just would like to know whether maybe I'm missing something important about why people does this... is only to be the easiest mode to gain the reward from Tyriel NPC or what? really is distasteful for me 'cause I never can do other bounties, all people leaves always.