Its because its harder. Its not guaranteed. because most of the time it is deserved. Doing the frankly easy weekly heroic for coins does [i]earn[/i] you the coins that buy you weapons so that is earning. However not only is getting a drop also earned, its harder to get therefore is more "earned."
Lets first look at some facts. All the events that consistently drop exotics are late game, hard content. Heres a list
VoG
CE
Nightfall
Even though none of these are particularly hard they are all harder than the weekly.
The vast majority of exotics come from here, and though you can get exotics from engrams and strikes/crucible, think of how low of a chance that is. Ik there are loads of stories about exotic drops, but thats only because those are rare enough to share. No one posts about the vast, vast majority of strike/pvp rewards are crap and so no one talks about them, giving us the illusion that exotics aren't that rare from theses sources.
Even if people got them from a strike or control match, much of the time it's a player who has hours put into the game, so then its a question of which is earning; a 30 minute weekly or all the combined hours of playtime.
It could also EASILY be argued that xur isnt earning either, but thats a thread for another day.
Heres a story that sums up why its earning. I was playing crucible with a bunch of great players. I wasn't doing too good, only got one kill. At the end of the match i, with a 0.0076 K/D, received the holy grail of weapons, Gjallarhorn.
I instantly got some hateful messages. I was reading them when my mom walked in. She read the messages and got scared, she then said "You're movin' with your Auntie and Uncle in Bel Air."
I whistled for a cab and when it came near. The license plate said fresh and it had dice in the mirror. If anything i could say this cab was rare. But i just thought, "Nah, forget it. Yo home to Bel Air!"i pulled up to the house at about 7 or 8 and i yelled to the cabby yo holmes small ya later! I looked at my kingdom i was finally there.
To sit on my throne as the prince of Bel Air.
WAAA TRENDING WOOO
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And here we witness a pitiful Desticle trying to earn his way into the flood. Little does he know; the flood is rng as a physical entity. As a whole we can all -blam!- punch him in the groin and send him crawling back to the wretched pigs he calls "Guardians"