The standard error of a proportion will be the largest it can be for a given N$N$ when the proportion in question is 0.5$p=0.5$, and gets smaller the further the proportion is from 0.5$0.5$. I can see why this is so when I look at the equation for the standard error of a proportion, but I can't explain this any further.
Is there an explanation beyond the mathematical properties of the formula? If so, why is there less uncertainty around estimated proportions (for a given N$N$) as they get closer to 0$0$ or 1$1$?