In the MCU, Thanos had been searching for the Infinity Stones for a long time until he obtained them in Infinity War. Since 2011’s Thor, Marvel has been hinting at the Mad Titan’s existence. It was only in Infinity War that it became clear that he was after all, six elemental crystals. This was so despite him appearing in a number of films since then.
Since they first appeared in the post-credits scene of The Avengers, fans have assumed that the Infinity Stones will play a role in the series’ long-term storylines. To put it another way, Marvel Studios officially started the Infinity Saga’s conclusion in Age of Ultron when Ultron, not Thanos, decided to search for the Infinity Stones himself. Since he was already working with Loki and Ronan the Accuser as surrogates in the MCU, it’s likely that he was even more motivated after Odin’s death to collect the gems. However, it is more likely that his efforts to acquire the crystals predated his appearance in the universe.
Even though Thanos is the main antagonist of the Infinity Saga, there is still a great deal of mystery surrounding him. According to what we know, after Titan was destroyed, the villain embarked on a mission to rebalance the universe in order to prevent it from suffering the same fate. Gamora’s world, Zen-Whoberi, was still under the control of his minions, who had been tasked with killing half the planet’s population.
In other words, Thanos sacrificed his so-called “daughter” on Vormir roughly 20 years after Gamora was abducted at the age of 10. Thanos already had the Mind Stone by the time of the events of 2012’s The Avengers, so it’s safe to say that he’d been on the hunt for the gems for at least 15 years before he finally found them all in Avengers: Infinity War.
Several factors influenced Marvel Studios’ decision to keep Thanos and his Infinity Saga arc under wraps. As a result of this, Joe and Anthony Russo were forced to focus on only the most important aspects of the villain’s story in Avengers: Infinity War, as they were already responsible for bringing the entire MCU together.
There were many who felt that Marvel should have given the Mad Titan his own origin story instead of cramming it into one of their most jam-packed films to date, even though the 2018 blockbuster was a critical and financial success for the franchise. It’s possible that the studio did this on purpose to avoid further disruptions to the MCU’s micro-continuity. Certain timeline details don’t add up in the overall narrative of the universe. As the franchise expands, Kevin Feige and his team tend to avoid specific dates.
In any case, Marvel Studios could go back and revisit this in the future. The MCU can revisit the Infinity Saga, including Thanos’ quest for the Infinity Stones, through a prequel or through scenes in future projects. Eternals, in which he meets his half-brother Eros for the first time, spurs the studio on to action.