I think they explained it in the opening scene on the Asgardian ship with the following piece of dialogue: Ebony Maw: No. Let him have his fun. Thanos IS that strong, he just wants to have fun. He demonstrates some of the power of the Infinity Stones throughout the movie but purposefully doesn't utilize it to the extent he could. As an example, the fight on Titan. Dude literally just summons a fucking moon. He could very easily have just had the moon crater the whole battlefield as a whole. Instead, he broke it into bite-sized chunks and went meteor storm. He's playing with his food. That's why he took his armor off in that first scene. He wanted to prove that HE could defeat all. That he had the conviction to do what was necessary.