Priviledge and balance
So apparently, an entire team can go all AD melee assassin. They can go 0/100,000,000 in their lane, they can make every possible fuck up and give the enemy every possible advantage until all three inhibitors and both nexus turrets are down, but it's okay. They have late game [citation needed].
Meanwhile the support has to overclock themselves to an absolute extreme just to try and keep one player's head above water, to the point that even the most basic shit everyone takes for granted, like warding and recalling, is the most supreme of supreme luxuries that they can't indulge in.
And if they don't build one item, or they don't build one item fast enough, suddenly it's trolling, suddenly that's what made every lane fail, suddenly that's intentionally feeding. One item on a role that you treat like shit and pretend is nothing but a playable minion and a ward mule does not unfuck every mistake you ever made. Pretending otherwise or desiring otherwise would imbalance the game in ways you cannot even begin to ever hope to imagine.