As much as LFG tools should be in game, if you can't be assed to use a LFG site for 5 minutes, sorry, but I can't trust you to be in my fireteam/ party and not be a detriment and waste everyone else's time. If that you're not willing to commit a modicum of effort and time towards a challenging, end game activity, then maybe said activity isn't for you.
It isn't meant to be a casual romp, where you can be on your phone or watch Netflix simultaneously. It demands a considerable amount of time, concentration and coordination with your team.Your average player simply isn't willing to exert that sort of effort and commitment. They'll leave just as fast as they came in and you'll never finish in a reasonable amount of time, if at all.
With LFG, you can weed out the riff raff and invite players who want put forth a serious effort towards completing the activity.