MechaPilot
Explorer
I don't have to. Low int = low ability to reason. It's clearly lower than average, so a player running around with a low int PC reasoning everything out and solving all the riddles is roleplaying badly. If you're going to play low int PC as highly intelligent with a penalty, you are clearly doing it wrong. I don't need an exact number to be able to say that confidently and correctly.
Low INT can equal a low ability to reason. Whether it does or not is certainly not guaranteed because INT is described as having more facets than just reasoning.
Even if we take it as a certainty that low INT does equal a low ability to reason, quantification is still required or else you have no basis for determining whether or not a character is portraying a character as smarter or dumber than the character is, which is apparently something that matters somewhat considerably to you. Now, I have quantification for my position; it comes directly from the rules of the game, and none of your protestations can change how math works.
Again, feel free to do as you please in your own games, but the simple fact of the matter is that 5e INT scores do NOT equate to tenths of the characters' IQ scores.