Question

why are electron carriers present in p-type semiconductors

Answer 1

First, all carriers in semiconductors are electrons - vacuum (holes) doesn't transfer charge. Hole its just a way to describe electrons movement across valence band which is almost filled by electrons. Electrons in conduction band of p-type semiconductor could appears by three ways:

background donors doping (or thermal donors)

injection from n-type area

band to band tunneling or direct thermal generation