Because there was a deceptive map of the ancient Khmer empire that the French had written to prepare for the conquest of Siam.
But it was stopped by Russia and many other European friends when King Rama 5 visited Europe and Russia.
This made France reluctant to do such a thing until now. There are only Khmers who still have the courage to use this map as a teaching curriculum until today.
This makes Khmer children in this era claim every Thai matter without knowing the universal truth. and with the delusion that the Khmers once ruled the land around Suvarnabhumi, especially the land related to the Dvaravati period.
The ancient Khoms (who were not Khmer slaves at all) until the arrival of the Siamese which was not true at all.
The ancient Khmer Empire never actually existed in this world.