Muito possivelmente porque ele orquestrou as coisas para que as pedras fossem reveladas.
No final de Age of Ultron , quando Thor for embora, ele diz
Thor: I have no choice. The Mind Stone is the fourth of the Infinity Stones to show up in the last few years. That's not a coincidence. Someone has been playing an intricate game and has made pawns of us. But once all these pieces are in position...
Fonte: Transcrições Wikia > Vingadores: Era de Ultron
considere também que foi Thanos que deu a Loki o Cajado que tinha a Pedra da Mente e contratou Ronan, o Acusador, para encontrar Power Stone, ou pelo menos o Orbe sabia que estava nele
Ronan: I promised Thanos I would retrieve the orb for him. Only then will he destroy Xandar for me. Nebula, go to Xandar and get me the orb.
Ronan: The orb is in my possession, as I promised. [Korath holds up the orb in his hand]
Thanos: Bring it to me.
Ronan: Yes, that was our agreement. [Ronan takes the orb from Korath] Bring you the orb, and you will destroy Xandar for me. However, now that I know it contains an Infinity Stone, I wonder what use I have for you.
Thanos: Boy, I would reconsider your current course. [Ronan opens up the orb and looks at the Infinity Stone inside]
Korath: Master! You cannot! Thanos is the most powerful being in the universe.
Ronan: Not anymore. [suddenly Ronan embeds the Infinity Stone in his warhammer and absorbs its power into his body] You call me "boy!" I will unfurl one thousand years of Kree justice on Xandar, and burn it to its core! Then, Thanos, I'm coming for you. [he ends his call to Thanos]
Fonte: Transcrições Wikia > Guardiões da Galáxia
também em Avengers: Infinity War , é revelado que
Thanos had sent Gamora out to find the Soul Stone and after capturing and torturing Nebula, learned that she had found it
enquanto Power and Mind é facilmente explicado, nós não sabemos exatamente como ele sabia onde a Pedra do Espaço estava no momento em que a procurava ou que a Pedra da Realidade estava em posse de Tanhereer Tivan, mas dada a declaração de Thor em < em> Age of Ultron ele pode ter orquestrado algumas coisas para que ele pudesse encontrá-las de forma mais confiável, como se opor a ter deixado o Tesseract na Terra e o Éter escondido pelo pai de Odin, Bor Burison.
como para a Pedra do Tempo, enquanto não sabemos se ele tinha alguma mão com o Olho de Agamotto terminando em Kamar-Taj, no entanto, eu suspeitaria que ele ou a Ordem Negra sabia que ou
a) Attacking the New York Sanctum would lure out Strange with the Time Stone
and/or
b) One could get to Kamar-Taj from the New York Sanctum as the Black Order landed in New York rather than Tibet
existe também a possibilidade de que os Stones reajam uns aos outros à medida que se juntam como
Vision's Stone start to react with him commenting that it was trying to warn him and Thanos only needed a couple of stones, the ones he knew where they were, in order to find the rest. however this might have been a unique property of the Mind Stone as Thanos still needed Gamora to reveal to him the location of the Soul Stone