Whoo boy. A lil’ too much supposition goin’ on here.
Okay, first, as already noted, bullets come in all manner of shapes and sizes. A .45 ACP FMJ (full metal jacket, IE, a military “ball” round) is a sort of tall dome shape: Flat base, cylindrical bottom half, rounded nose. It’s perhaps only half again longer, if that, than it’s diameter.
On the other hand, you have something like a “spitzer” bullet from a .30-'06 or similar rifle. This is the classic long, thin, severely tapered and nearly sharp-pointed bullet. It’s over four times longer than it’s diameter, and most are “boat tailed”- there’s a truncated cone base.
Why the difference? Simple- the .45 isn’t meant to be a long-range round. Decent accuracy inside fifty yards is fine. The rifle round, however, is intended for maximum accuracy and retained energy at extended ranges. Therefore improved aerodynamics is required.
Second, and most important: While the “football” shape may indeed be one of the more aerodynamic, a bullet needs what’s known as “bearing surface”. This is the straight-sided portion of the projectile that actually comes into contact with, and is lightly swaged into the rifling grooves of, the barrel.
To a certain extent, the longer the bearing surface, the more stable the projectile will be. (Too long, of course, adds additional drag and wear.)
The .45 can get away with a short bearing surface due to it’s relatively slow velocity (700 to 900 fps) and limited range. The rifle bullet travels much faster (2,300 to 3,200 fps, give or take) and usually has a faster “twist”, or steeper rifling (usually expressed in inches per turn- a 1:16 will make one complete revolution of the rifling grooves in sixteen inches.)
Thus, the rifle bullet needs more bearing surface for proper stabilization, and to a lesser extent, to keep the bullet from being blasted straight out before it can “grab” the rifling and begin to revolve.
So there’s a limit to the shape of the bullet- plus, as noted, the gasses from the propellant need to have a flat surface to push against. Rounded bases tend to be leak-prone and inefficient.
Why not make the cartridge longer? Well, they do. Frequently.
At the turn of the last Century, we had the .38 S&W short. To get more power, it was lengthened slightly to the .38 Long. Later still, in the thirties, it was lengthened again to make the .38 Special. By 1958 it had been stretched yet again and given modern powders to make the .357 Magnum (which, despite the name, still uses the same .357" bullets as the .38s have since they stopped being heeled rounds.) More recently, it was stretched yet again to make the .357 Maximum.
The .44 Magnum is a stretched .44 Special, itself a decendant of the .44 Russian. The .45 ACP was stretched and made into the .45 Win Mag. Ad nauseum.
However, case volume limitations still exist, besides the aforementioned bearing surface question.