because thanos did not attack before the earth

As we know that for decades it has been waiting for the right moment to destroy life in the universe the only people who could stop it nowadays would be the avengers and their allies but you would think that they were afraid of others that this is the reason why the had attacked before the avengers existed there were other people or very powerful beings who could have faced thanos if this became a threat the old woman the teacher of doctor stranger this character was incredibly powerful had in her power the gem of time n had been easy for thanos to take it away as we know that this character died in the movie of doctor stranger
Another character is ego in the living planet this character is a celestial of the most powerful identities in the universe being father of star lord as knowing star lord could touch and use the gem of power was partly a celestial and for being son of ego

This means that ego was able to also use the gem of power and even equalize its power. This would also have been inconvenient for thanos if it had been sele versus in another

Odin this at one time was an unstoppable warrior would be a great threat to Thanos we also knew that he had a connection with the Odin and Hela tesserat

Hela when she saw the teserat knew it was very powerful this means that they had also interacted with him before with a gem of reality and as we know that in thor ragnarok Odin died leaving loki and thor as his successors

To the others who are afraid of thanos is the great teacher that we saw in thor ragnarok and the collector

Since the great master did not show to be a threat nor the collector this means that the only ones who feared him thanos were the ancient people who could deal with thanos now they were dead at last in dying would be Odin therefore this elder of the universe had dead is the way clear for thanos this is the reason whyosos decided to attack after and not before since the threat against him had disappeared