It is a fact that some cats will not tolerate newcomers to their environment. We had a Siamese named Jade (I loved her dearly, still do in my heart) and I brought home a new gray kitten - we named him Tomas. Anyway, they lived together for years - never got along. Jade never really forgave me - we still had a strong relationship but Tomas was anathema to her. He used to antagonize her too of course. She would get under the covers to sleep at night; Tomas would settle himself right on top of her on the bedspread. Tense situation.
I have had the best luck getting the cats at the same time. Not always from the same litter either. I had a brother and sister who didn’t get along that well until they were six, seven years old. Then for some reason, they totally bonded, for the rest of their lives.
But I have seen cats go into homes with other cats and do just fine. I believe the personality of the cat plays a huge role. Also the people, environment. I think animals are more complex than people give them credit for - I agree we project humanity onto animals but we equally often tend to diminish them into ‘thingness’ almost too. They are not dolls, plants. It is important to give an animal time, space, patience, all that when it enters a new environment - just like you would a person. Relationships grow. This is very true of cats.
Age can be an issue.