LNBF is made of horn antenna, waveguide, half/quarter wave dipole and some circuitry on PCB. I know Horn antenna is used to match impedance between waveguide and free space but my doubt is, why we dont use half/quarter dipole antenna directly instead of using horn antenna, waveguide and all?
Dipoles are linearly polarized. Most satellite signals are circularly polarized, this allows satellites to rotate, eliminates alignment issues on the ground (instead of horizontal and vertical linear polarization you would end up with n/s and e/w linear polarization, or whatever axis was used). Also, signals can get rotated when traversing the ionosphere which does not affect circularly polarized signals (they still get rotated but the hand-ness of the circular polarization does not change).
Also, many satellites transmit separate signals with right hand and left hand polarization. A dipole would mix both signals which is not desirable.
There are some satellites with linear polarization (probably mostly older satellites in geosynchronous orbits).