It's not that you're stealing kills from the adc. Nobody says you can't play a middle ground between hard carry and supporting the team. It's just that when you have gold, it's better to try to snowball that gold lead through carrying than trying to support your team. It's why you aren't buying mikhael's crucible for your adc in every role.
Besides, it's not like snowballing your own position is actively hurting other lanes. A kill in any lane isn't just about the gold from the kill, but the gold difference you can generate while they're dead. If you kill a lane and don't push into turret, you're giving the enemy room to make up the gold deficit they accrued while dead. A wave pushed into the turret is a wave of cs that the enemy isn't getting, which is ~120 gold. Assuming the person you're pushing with can cs, that means you should generate ~240 gold difference just from pushing the wave, let alone the kill gold. And that's excluding the XP deficit as well, which has the most impact in the early game, where you're most likely to generate noticeable gold and level advantages.
Basically, playing greedy isn't wrong so long as you aren't being blinded by the greed. A selfish purchase can indirectly benefit the rest of the team if you can use that selfish purchase to secure more benefits for your team. That's essentially the entire premise of the ADC position to begin with, but that mentality can translate into any other role.