It's a core weakness of Helldivers 2. Since the game is no longer restricted to having all players within a tight radius (unlike the first game), the game allows for splitting up.
The only task that demands cooperation is the optional vault doors. All the other main objectives and side objectives can be solo'd. Terminid Chargers could be an excellent focal point for cooperation but flanking a charger doesn't help kill it faster, it just lets you shoot the bottom half of its rear for 30 secs until you cause it to begin bleeding out.
It is safer for the rest of your team if you bring answers to all the problems on your own because you could be waiting for anyone to respawn. And if you have an answer to every enemy type then you might as well split up since you don't need another pair of hands to complete any objective. Having multiple guys on the objective does not make it go faster, so it's better to start countdowns simultaneously.
I'd guess it is because the most popular way to acquire games for Citra was Nintendo's own servers