How can one religion split into so many sects while each claims to be the “true” one?

I’ve been thinking about something lately and I’m genuinely confused about it.

Why do religions split into so many groups and sects over time?
For example in Islam you have Sunni and Shia, then inside Sunni there are different madhabs, and inside some groups there are even more branches and interpretations and for Shia as well.

The same thing happens in Christianity and other religions too.

If religion is supposed to come from one truth or one God, why are there so many disagreements between believers themselves? And how does an average person know which interpretation is the “right” one when every group believes they are correct?

I’m not trying to attack anyone, I’m honestly curious and I’d like to hear how atheists or ex-religious people think about this.

🫂🫶🏻