Because Bungie. But if you must ask, I suppose I'm one of the ones to properly explain it. "Mythic" "Legendary" and "heroic" were titles given to members of the forums on the older "version" of this site. Mythic members, like Helveck and myself, were given a high trust-rating by some mysterious system Achronos made or conjured into being with demon witchery. For the most part it meant nothing. But over time people saw it as a sign of esteem and prestige. The blue bars (mythics) usually posted better things and were held to a higher standard. Usually due to the lack of bans needed to get mythic status, honestly. It became a running joke that non-mythics weren't good enough. But, as to why specifically 7 years? Don't make me put you in a giant slingshot and fling you into the sun.