[empath75]

e isn't really a fundamental physical constant and doesn't show up very much in famous scientific formulas. It's a number that has very convenient properties for performing calculus operations and shows up in that context. pi isn't really an important physical constant, either, but it shows up a lot because of two things --- it's useful for calculations involving cycles, and because things being spherical or circular is a useful simplifying assumption (and also because things particularly in astronomy end up being spherical or circular (or nearly) quite often in reality).

Neither of those things are sort of empirically measured, and in any formula where they show up, you could theoretically absorb them into other constants -- and in fact the Einstein gravitational constant does exactly that -- it's defined as (8*pi*G)/c^4, absorbing pi into newton's gravitational constant (for historical reasons, when using plank units, they set G and c to 1, so it ends up just being 8pi -- _reduced_ planck units set the whole constant to 1). It's just frequently easier to separate out e and pi for the purposes of actually working out the math.