They can say the same about me, right? I have so many communication apps on my phone, why do I draw the line on Facebook Messenger?
Most likely you're the only person they know on Signal and it makes more sense to them that you move to Facebook rather than moving their entire friend-sphere into Signal.

Sure but the social aspect in games is often pretty toxic.
I played quite a lot of Dota 2 and while communication and cooperation greatly improve your chances of winning... More often then not it was just someone raging hard over minor mistakes.