You did see what happened on Titan, right? They could have easily beaten Thanos. The only thing that stopped them was Thanos having all the stones. If the Avengers worked together, they could have easily beaten Thanos, who in this case wouldn't have had the time or mind stone.
What happened on Titan was a specific combination of people that were not on Earth at the time. Part of why that would have worked if Quill hadn't screwed it up, is because Mantis was subduing Thanos. Thanos had the reality and the power stone at that point, as well as the space stone. Three infinity stones. The reality stone alone may have been enough to foil their attack against him.
