I thought the requirements would be much stricter. You're missing the point a bit, TC, and by saying "I don't need to know how to play a tank or healer to teach someone how to be a good DRG" you're also assuming that the player you're mentoring only wants to be a DRG. Leveling the three roles to 60 makes you a more well-rounded player, and guarantees that you'll have first-hand experience with how the 3 different roles with their full toolkit work in relation to one another.
Yes, it's true that you can't give specific advice to a would-be SCH if you have only have a WHM at 60, but the core fundamentals of healing are the same across all jobs, same goes for tanking, and DPS, the difference being what skills they have in their toolkit in order to at the very least meet the minimum requirements of their roles.
It's also worth noting that a player who has leveled a tank, a healer, and a DPS to 60, has also or will level other jobs. I'm sure that people like this exist, but I've been playing since 2013 and to this day, I've yet to meet someone has JUST has one DPS, one healer, and one tank at max level at least 6 months after the current level cap; if they have one of each, they're extremely likely to have others up there as well
Yes, just leveling a job to 60 by no means guarantees that they know how to play it, but it's the basic requirement to do endgame content, so it's probably SE's minimal way of excluding players from giving advice who haven't experienced level 60 content and understand how their role works once you have your entire toolkit at your disposal; you can't give advice if you don't know what you're talking about.
TLDR; if you want to be a mentor that badly, go and meet the very lenient requirements. If it's too much work, you don't have to be a mentor. Complaining will get you nowhere.
quick edit:
No, you really don't. In fact, you've barely scratched the surface.