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human
[MCU Infinity War] Why could Xandar, Loki, or the Collector not stop Thanos?

When Thanos destroys Xandar and takes the power stone, he has none of infinity stones in his possession. So it was just his army versus the HQ of a galactic empire. How is it that they so easy fall to him? Or how does Xandar not figure out how to use the stone and stop him, or at least ship it off so it doesn’t fall into his hands?

Thanos then attacks the Asgardian ship housing Loki and the tesseract. This should have been the power stone vs the space stone, why did Loki not even attempt to use it upon seeing Thanos wielding his own stone? You’d think the space stone + Thor + Hulk + Heimdall could do anything to slow down Thanos. They literally just defeated Hela in the events of Ragnarok, so they’ve shown they can stand up to a universe-wrecking villain. 

Then, when he travels to Knowhere, it is the power stone and space stone vs the reality stone. The Collector knew of the power of the reality stone, why was he unable to put up any sort of fight to stop him?

Even when Thanos sends his children to get the time and mind stones, how is it that they can overpower competent wielders of stones? by this time Strange has had plenty of time to study and master how to use the time stone, but barely puts up a fight. Vision is literally the embodiment of a stone and doesn’t throw a single punch. 

Is Thanos just the only one capable of intentionally using the stones to the fullest of their powers? It just seems like it should have been harder for him to be stopped at the beginning of Infinity War instead of instantly being able to acquire stones to instantly get three of them in thirty minutes. Not only that, but two opposing stones got taken out on earth by two people each! How is this supposed to make sense?
gpt
Out of all of your examples, the only competent user of an infinity stone was Dr Strange. Even without any stones, Thanos and his army are incredibly powerful. Thanos on his own took down Thor, Cap with mjolnir, and Iron Man. Thanos' army also seemed to be doing considerable damage against the combined forces at the end of End Game. Ignoring the stones, if not for Scarlet Witch or Captain Marvel (Dr Strange was out of the fight), Thanos would have probably defeated everyone else.

It isn't too crazy to think that Thanos could either assault Xandar, or have his powerful assassins sneak in and grab it.

As for Loki, he knew he couldn't beat Thanos, even with the space stone. His best bet was to lie about it and hope Thanos leaves them to go check the ruins of Asguard. Loki only revealed it when his brother's life was on the line.

Thanos and his forces are just THAT strong, and Thanos has a gauntlet specifically designed to allow him to use the stones.