The incentive to mentor is long-term. It benefits the company later on, and loyalty is an asset for the person doing the mentoring (his proteges might move with him if he gets sacked). Unfortunately, few people think this far ahead. In the short term, mentoring is an expense.
Candidates, especially young or recent graduates, are not. If you, an exec, hone a pitch on a jaded 40-something VC manager with 20 years of industry experience, and then use it umodified on a 20-something candidate with 0-1 years of industry experience - you are not being truly fair and honest.
And yes, the candidate is likely to discover that a year later, feel cheated and abandon you. Tweet about him being a "flake" if that makes you feel good; the blame lies solely with you.