Why macroscopic bodies should exist as wavepacket? Based on my understanding, we assume that the electrons, exist as wavepackets in the solids while deriving the transport equations for transistors, we create wavepackets out of momentum eigenstates for a free particle and say this is what a classical state should look like, but why this can be the only case??
For example, cannot we have a free particle with energy on the scale of macroscopic bodies and still it is a momentum eigenstate? And why a single wavepacket, can't we have two wavepackets in opposite directions such that once i see the particle to my right and on next blink, it is magically to my left? (or will it collapse to one of the either wavepackets as soon as i see it since that constitutes a measurement?)
Is there any phenomenon that forces particles with macroscopic energy scales to exist as wavepackets in both x and k space?
 A: 
Is there any phenomenon that forces particles with macroscopic energy scales to exist as wavepackets in both x and k space?

In modeling nature with mathematics one has to have clear definitions.
The "wave" in a general wave equation, refers to a sinusoidal solution
The "wave" in a quantum mechanical equation that is used to model a particle is in the probability distribution, but as far as a space time identification for modeling  a particle, is useless, because a plane wave solution , which is the solution with no potentials, gives an infinitesimally  small probability to find the particle at  a specific (x,y,z,t).
The wavepacket model allows for localized motions in fluids and sound,

Since the traveling wave solution to the wave equation
is valid for any values of the wave parameters, and since any superposition of solutions is also a solution, then one can construct a wave packet solution as a sum of traveling waves

This method also can be used to form a high probability solution for finding a quantum particle at (x,y,z,t) and thus localize it.
To model a track for a free electron, one can have a wavepacket of the plane wave solutions representing the locality of the electron, a high probability of finding the electron there with that energy. This will be useful in solids , as you state.
The only "phenomenon" is that one cannot use simple plane wave solutions  to represent a quantum mechanical particle, because of the identification of the wavefunction $Ψ$ with the probability (the only prediction of quantum mechanics) being  $Ψ^*Ψ$.
