It's because most people say 'ee' with the space inside their mouth flattened to nearly nothing. Try 'ee' this way:
-inside your mouth, open tall all the way back into your throat. Make it into the greatest possible space without lowering your jaw (for me it's on the way to yawning but stopped just before the point of no return).
-your lips, jaw & cheeks do nothing at all, don't go wide like a toaster; they should flop if you say a breathy 'p' sound.
-while gently holding open that space with 'inside muscles' in your mouth/throat, use minimal muscles (no jaw, no cheeks, no lips) to shape the ee sound.

Very different from the way we usually learn to speak, but go ahead, try it! You won't look funny at all with the visible muscles relaxed. Yes, it does take getting used-to. And once you do (it took me 6 months of messing about every day to become fairly automatic) it's fully resonant; you can add as little or as much nasal quality as you like. It makes ee nearly as rich as ah, oh, oo etc.

Well, a picture would be worth a thousand words, wouldn't it?!