Why are the rockets not razor sharp
Because then the payload fairing would spend a lot of mass for space that wasn't useful to enclose a payload.
The skinnier the rocket, the worse its volume-to-surface-area ratio, increasing its weight for a given capability. Additionally, some rockets like SpaceX's Falcon 9 are already skinny enough that flexion of the rocket body is a problem.
and tamper off at both ends
The back end isn't tapered for a number of reasons, discussed in an earlier QA.
Would making a rocket more aerodynamic allow it to be more efficient flying
as a "Kármán plane"?
Less so, if anything; a longer, skinnier rocket is less able to make use of body lift. In any case, orbital launchers don't hang out at the Kármán line; they get past it and well above it in a big hurry.
For large launchers, aerodynamic drag produces only a small performance penalty -- only on the order of 1% of the ∆v produced is lost to drag. Making them pointier would increase weight or decrease payload volume to little benefit.