Because they disagree about the Messiah, a predicted part of their religious myths. Christians believe it is Christ; Muslims believe it was Mohammed; and Jewish people believe they were both prophets only, and the messiah is yet to come.
(To add to this Jesus and Mohammed both preached love and wanted people to get along, which was quite a change from their original, ‘an eye for an eye’ teachings; a complete contradiction in fact. Hence if you believe the new messiahs’ teachings you have to change your perspective considerably, some people get confused and think they can cherry pick from the old teachings and the new ones).

you are almost at an important point in your thoughts and I hope you realize it. we are all going to the same destination. none of this petty bullshit we do as humans will change that.

what does change, or should, is US. just as our minds develop new connections (learn) up to the day we die ... we change our internal narratives throughout our lives. we measure it daily, hourly, minute by minute against the evidence that unfolds in front of us and are SUPPOSED to tailor our narrative to meet those events. churchies tend to absorb this one narrative thats fed to them from birth and defend it from all contrary evidence. that is ignorance. literally. and the simple fact that all of these groups have a different view means potentially all of them are wrong. we known not all of them are correct.

In my view we change and revise our narrative based on the following rules:

1. seek truth (satyam bhaja)
2. create order (kalpitam kuru)
3. avoid discord (adharmam tyaja)

This is "the funnel". It guides us all to eventual truth.

..cuz there are no gods… But really, if they’ve got their ‘one god’ pegged, described, and worshiped - leave the rest of us alone!

Glad you’ve the confidence to speak your mind on facebook. I don’t, which is another reason I come here. ‘Friends’ know, but I only let it out in trickles. Here I can gush