This is a most sensible question. But why does it not happen? Why don’t they sit together and solve their differences?
The reason is that two of the three religions, Islam and Christianity, demand blind belief in only their story about the true God and his creation.
Muslims need to believe what around 1400 years ago Prophet Mohammed supposedly said and what is written in the Quran. And in some verses, for example 8.39 or 9.29 the Quran says that Allah wants all to become Muslims, and Muslims even need to wage war to reach this goal.
On the other hand, Christians need to believe what around 2000 years ago Jesus Christ supposedly said and what is written in the Bible. And some verses, for example John 14:6 or Matthew 28:19-20 imply that all need to become Christians.
Since blind belief is demanded of ‘good’ Muslims and ‘good’ Christians, even the most compelling reasoning, or sitting together and suggesting a compromise will not work, because they need to follow the instructions from their book – at least as long as they believe that their book contains the Truth. However, when they get doubts (and actually many got doubts), they are no longer ‘good’ believers, but probably use their (God-given?) reason and intelligence again.
In contrast, Hindus are encouraged from childhood to use their reason and intelligence. And they cannot understand why Muslims and Christians don’t also use it and come to the conclusion that the merciful Almighty surely will not send billions of humans into eternal hellfire, only because they call Him Brahman or Ishwar and not Allah or Godfather.
One more important point:
Muslims and Christians wrongly believe that Hindus are idol-worshippers and worship many different Gods, while in contrast, they worship the true God. This belief that Hindus are idol-worshippers, is the main reason that ‘good believers’ look down on Hindus, despise them and even killed them. Idol-worship is considered in Islam and Christianity as the greatest sin. It means to worship other Gods besides the true God, which seems to make the true God so angry that he will burn those sinners for ever in hellfire.
If Muslims and Christians would realise that, according to Hindu Dharma, all the different Deities of Hinduism are different aspects of the ONE Brahman, they could get over this wrong belief that Hindus commit idol-worship and get over their disdain or even hatred for Hindus. They would realise that Hindus, too, claim that the source of the creation is only ONE. Of course, there can be only one Source for this miraculous universe.
Judaism, Christianity and Islam, which came much later than the Vedas, agreed with the One Source of the Vedas, but they made the unimaginable Brahman into a personal God who has likes and dislikes.
Hindus actually have the clearest understanding of the ONE SOURCE of everything. The Vedas claim, only this one Source, which they call Brahman or Paramatma or simply TAT, is absolutely true. All else is Maya, appearing as true but is not fully true. The Upanishads teach “Brahma satyam, jagat mithya”.
The many devas of the Hindus are not separate entities. But we humans, animals, stars, etc. are also not separate entities. All is interconnected. The ancient Vedas postulated that this variety in the universe is in truth ONE. (Does it surprise that modern science agrees with this view?)
Unfortunately, on their own, the followers of Christianity and Islam can’t see this truth and Hindus still don’t try to make them understand, maybe because a few centuries ago, such reasoning with the wrong people would have costed Hindus their head.
Many Hindus know that their Sanatana Dharma (or Hinduism as it is called today) has a solid basis and is preferable to blind belief.
Maybe it’s now time to say it also aloud and let the world know about this wisdom.