This question goes to the heart of human nature, experience, and justification as a product of the human genome. If we slightly soften the ask, then there are plenty of examples of ideas that are near-universal, reasonable propositions, such as 'one and one makes two', 'up is the opposite of down', 'ants are smaller than elephants', and 'fructose and sucrose are sweet'.
Technically speaking, the quantifier ALL is very strong, meaning that somewhere on earth, if 1 of more than 7 billion people disagrees, there is no universal agreement. You then complicate by predicating with "rational people", whatever that may be. Even clinically insane people can use logic, no? But let's just tighten the proposition a bit by saying anyone who seems reasonably integrated into their society which allows us to use the lens of cognitive science and philosophy of mind within the context of a naturalized epistemology to examine cultures for universals and particulars.
First, if you accept psychologism as a thesis, then you are in good company, and can take note of the proposition that genes among humans are nearly identical. In fact, you share 70% of your DNA with marine worms, and with chimps, closer to 98%. From a statistical, genomic picture, it is defensible to claim that you are almost genetically identical to your closest relative. Should it be a surprise then, that since you share so many genes with your fellow Homo sapiens, that you should have the same rational ideas? This is a fundamental thesis of a philosophy of mind that undergirds embodied cognition and is seemingly displacing other theories of mind among cognitive scientists.
Steven Pinker, the internationally famous psycholinguist, in his The Blank Slate, devotes an entire book outlining exactly how similar and different we are and has a wonderful appendix devoted to Donald E. Brown's anthropological work on human universals. The appendix contains a list of over 250 universals of human state and behavior and gives a legitimate characterization of the human condition.
But you are more interested in ideas than in behavior, and to that, it is possible to draw your attention to the metaphysical presuppositions of cognitive semantics which embraces a wider vision of human meaning allowing to theorize (somewhat pre-scientifically) all human thought as emerging from a similar foundation. One of the seminal works of the field is Women, Fire, and Dangerous Things: What Categories Reveal about the Mind by George Lakoff who in conjunction with other linguistics from the same faction of the linguistics wars has argued a metaphysical position called embodied realism.
Ultimately, why "rational people" agree on "one and one makes two" is a function of the idea that the brain as we understand it has universal anato-physiological function, and thus supports a variety of functionalism in philosophy of mind.