Because it makes you sound important. If you have a mentor you are important enough for special teaching. If you are a mentor you have some special guarded tao of your field a mere mortal cannot understand.
Same reason every Cxx on the planet says they "talked to their mentor" about whatever. We used to say "a friend" or "a colleague" but like all things in SV it gets turned into word salad. Another word that drives me up the wall is "learnings". It makes me feel like I'm taking crazy pills every time someone says "we took back some learnings".
