Because they are not allowed to think for themselves, they are guided and obliged to say what others want and in most of the cases that's the church.
Look at developed countries, where people can decide for themselves what and who to believe. In those countries people can accept the fact that some of us are different, it's a normal thing, has been for ages. Denying it, punishing it did nothing good, no results were accomplished.
I truly hope this brainwashing and hatred can stop one day, the sooner the better! People need to open their eyes, they need to learn instead of believing blindly, which leads to manipulation!
It is a question that many of us ask ourselves, because it seems hard to believe that in the 21st century there are many individuals who are intolerant of others.
Intolerant people express their ideas in different ways, for example: while some do not accept homosexual people and remain silent, compared to some who do not accept them and respond aggressively. Making it clear that they still live with a primitive (or perhaps atrophied) mind supporting prejudiced ideas.
I think everything starts at home with the values instilled by the parents, where most come from a time full of prejudices and stereotypes. Or because they are fanatics, which is not a good thing, because they cling to an idea that favors the formation of prejudices and discrimination against other groups of people.
Everything would be through the primitive era where it was believed in a single way of life, a woman with a man created a family, while the man worked and the woman raised children, and so on until the end of time. Mainly because of religion, that's where intolerant people stand, because of their fanaticism.