Why didn't Thanos just use the reality stone?

If you're talking about the scenes that take place after he comes to earth, we could say it was hardly a fight. One by one all of the avengers were quite simply beaten by Thanos and the only ones he had to even remotely fight was Wanda and Thor.

1. Egoistic Thanos: Thanos already had 5 stones, he was already extremely powerful, he didn't really think of the avengers as a roadblock, he just wanted the 6th stone. So he didn't feel the need to put in that much effort though it would have ended things quickly. I feel that's the thing with major villains. They have so much at their disposal and don't use much of it.

2. Finale would be mehh: With the reality stone, he could just driven the avengers crazy. Or simply turned them to stone (for temporary time of course) but is that what we would have liked to seen ? The protagonists falling down like blocks ? Thanos has crazy power and to be honest, if he wanted the 6th stone, he would have got it in around 0.0098 seconds. I guess that's just not how movies are done. I feel the reality stone was not used to level out the fight till the end because that makes a better scene than Thanos altering reality (just an opinion though).

I typed the exact same question and typed it out before deciding not to post, dont know why.

Both good answers from Anu & eztephen - but just to elaborate on the writers intentions, they answered this very question in the audio commentary for Infinity War, stating that they saw the reality stone as an impermanent solution when used , the extent of it's power being mainly illusory as opposed to permanently changing the nature of reality in any given situation.

They also note that Thanos avoids murdering other characters unless absolutely necessary and in the specific case regarding the Knowhere encounter, one of his aims is to try and re-establish a connection with Gomora. As such, he knows that killing Quill, Drax or Mantis would only alienate her further, which is why he chooses to dispatch them using the reality stone, the effect of which is neutralised as soon as he escapes with Gomora via the tesseract powered wormhole (as Drax and Mantis are instantly able to pull themselves back together.

You are right that with the reality stone you can bend the reality based on you likeness. If you want to bend the reality on a planetary scale using the said stone, will be able to. But note that using an infinity stone alone has it's limits. If you want to bend the reality on a galaxy scale or universal scale, you gonna need the other stones to empower the reality bender stone.

Also, note that each infinity stones depend on one another to fully reach their universal range and full potential.

Time Stone empowers the Reality Stone. Reality Stone empowers the Soul Stone. Soul stone empowers the Mind Stone. Mind Stone empowers the Power Stone. Power Stone empowers the Space stone. And last, Space Stone empowers the Time Stone.

Together, they form the Infinity.

See the reference here: Infinity_Gems

There was nothing wrong with Thanos's strategy in Wakanda as he won quite easily

In constrast with the fearsome battle with Iron Man, Doctor Strange et al earlier in the movie, Thanos hardly breaks stride here as he dispatches the final obstacles preventing him completing the set of Infinity Stones.

The exceptions are Scarlet Witch (whose own powerful magic enabled her to oppose Thanos briefly) and Thor (who possessed the element of surprise and a unique Thanos-killing weapon).

The reality stone is powerful, but Thanos's plans require ABSOLUTE power

Using the Reality Stone alone would certainly have sped up Thanos's intention to cull half of all life in the universe. However, while other infinity stones were beyond his grasp, he might be defeated - or his actions reversed (for instance, by the Time Stone).

By acquiring all of the stones, Thanos acheives absolute power. This means that the effect of the 'snap' is immediate, universal, and irreversible.

These arguments are covered in much more detail here.

Infinity War directors answer that:

“We sort of used it as an impermanent solution to things,” screenwriter Stephen McFeely said.

“It’s illusory,” Joe Russo added. “It creates illusions.”

Indeed, while Drax and Mantis quickly returned to their natural forms after Thanos left their presence, the effects of the villain’s universe-changing finger-snap will last at least another nine months, until Avengers 4 hits theaters.

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Another answer for that is:

Even if that were possible, the Stones wouldn’t work in that universe or wouldn’t be able to come back. It was established in the comics very firmly that each set of Gems or Stones and their respective tool to use them only works within the universe in which they were created. There is a council of Evil Reed Richards wherein three of them from different universes gained the Gauntlet and the Gems only to not be able to use them to do anything else outside of their universe. There? They’re gods. When trying to take over the rest of the universe? The Stones and the Gauntlets are so much paper weights.

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