I think that has to do with the fact that not all children are able to learn/sense it at a young age. Telling parents their kids should be potty trained before the age of 3 could have the parent put pressure on the kid resulting in worse behaviour or aversion to using the toilet, lengthening the process even further. It's just easier to advice to "start at 3". Especially since nowadays more parents both work fulltime and will have less time to properly observe and guide the process at earlier age.