That reason has a lot to do with why they never seem to be able to realize that they have to teach children in the first place: no one seems to have taken the effort to teach them that in the first place. Given that John seems to have been a lonely social outcast who had a rather bleak and barren social life in his teens and since Elly seems to have been fairly firmly policed by a traumatized mother who saw every sign of initiative on Elly's part as a horrible sign that she would once again deal with a monstrous and vindictive tyrant, it seems to me that unlike their own children, they never actually babysat or learned how to deal with children as anything other than rivals for attention. Since they never picked up on the fact that children aren't trying to bust their asses when they ask questions or mess up, they're too busy feeling hurt and wounded to realize that they're the ones that dropped the ball.
We also have to remember another annoying fact about the two of them: they're both facets of the personality of an extremely troubled, angry, jealous and not very smart person. This is why John can spout that every day is Kids' Day and not see the blatant hypocrisy entailed by his using that lie as an excuse to award himself ego-gratifying toys in compensation for the alleged disadvantage he suffers under. People with functioning brains know that being an adult is freaking awesome compared to being a child but dumb-ass John can't see the perks that come from having control over his life. All he can see is the piddly crap that goes with it and bitterly envies Mike and the kids their alleged freedom they have because he's not just a glass half-full type, he's a "Who the [BOXCAR] drank my water?! I'll KILL them!!!!" type.
The end result is that John does actually end up teaching Mike something after all. He's teaching him that authority figures are lying sack of shit bastard idiots who all want to jam it to him out of pea-brained malice. This is why he thinks that Gluttson is a sociopath, Mira wants to eat his penis and why Divala had to go down for making fun of him. Every person falsely accused of wanting Mike to fail and be miserable so they could laugh at him for daring to want to be happy to owes this to John being a lying, hypocritical, self-pitying dickchoad who's too squishy inside to take crap like everyone else.