Because when the friend group goes on an outing, the planning is happening in an iMessage group chat, because it's a group of 15-20 people who all have iPhones, already have iMessage installed, and are not used to juggling multiple chat apps because everyone else they know has an iPhone.

I care about using iMessage because it's 1000% easier for me to set up blue bubbles compared to:

1. Convincing 15-20 people that it's worth all of them juggling multiple chat apps just for the convenience of 1 person (me) or
2. Having one of my friends relay every change of plans and update on who's coming from the iMessage group to me via signal.