Why didn't anyone make sure Son Gohan kept training?

[Kagari]

It was Gohan's responsibility to keep up his training. But does anyone here really think Chi Chi would've allowed it.

Since Son Gohan didn't even think to have asked any of his friends to have taught him the Multi-Form technique so that he would have been able to train the technique and improve upon it until discovering a variation of it that would have allowed him to have trained and studied at the exact same time, I have come to believe for a very long time now that Son Gohan is the type to never learn from his past mistakes. As he keeps on repeating many of them again and again without end.

You're exaggerating then since it only happened twice and since the second time he has learned from said past mistakes. He's the only one of the main Saiyans in Super to see any lasting development like that. You only need to see the latest manga chapter to see what I'm talking about.

[funrush]

I think Vegeta's probably the only one who actually expected someone stronger than Cell to pop up. Freeza was (as far as we knew) one of the strongest people in space, and Cell was created using the Saiyans/humans/Freeza. The only other known threat was Beerus and Vegeta probably assumed Beerus was unbeatable.

[Sadala Elite]

The "Gohan never wanted/doesn't like fighting" argument is extremely dumb for many reasons and it often sounds like an excuse for bad writing.

Characters in fiction (and people in real life) willingly do things they don't enjoy or want all the time for many reasons. Not only is it a potential source of good storytelling, it also helps character development. Future Trunks doesn't like being a fighter either, yet he never stopped training despite living in about 10 years of peace before Goku Black appeared.

Its makes no sense why Gohan, a character that's generally more heroic, cautious and responsible than Goku, to just completely slack off after the Cell Games, seeing how he's the strongest Z fighter at that point and therefore Earth main line of defense. Hell, Gohan himself was ashamed of himself for slacking off when Buu was on the verge of revival.

[ankokudaishogun]

Trunks was aware of the possibility of beings stronger than he was when he killed Future Cell.
He was also the last fighter on Earth.

Gohan didn't think somebody sensibly stronger than Cell could appear. While he DID slack off, he was still stronger than Cell, just not as much as he was during the Cell Game.
And there were other fighters aside him, especially Vegeta who did never stop training.

[kemuri07]

Honestly, it does make sense that Gohan would slack off. It's not a matter of whether or not he hates fighting that's the question, it's the fact that he just doesn't have the same drive as Vegeta or Goku: He doesn't fight simply for the sake of it. He fights to protect others or when he gets in his feelings. I mean all of DBZ it's Gohan getting into conflicts that are beyond his control. So it makes perfect sense that, while might have trained especially during his stint as The Great Saiyaman, I don't think he ever really considers the possibility that that someone stronger might come along. Especially when 8 years passed with little to no conflict.

Besides, fighting wasn't his priority anyways. It was always schooling, something that both DBZ sequels have been pretty damn consistent about.

[Gligarman]

One thing that got lost in translation is the fact that Gohan never wanted to be a fighter. He wanted to be a scholar. After beating Cell everyone, except Vegeta of course, felt he earned the right to live his life the way he wanted. One thing I’ve noticed over the years is that Toriyama seemed to move away from the idea that all Saiyan are battle-crazy tyrants. He’s since established that some simply don’t care to fight, which I appreciate because I never liked the idea of an evil race in fiction. Just comes off as lazy. That’s why I’m glad that in addition to Gohan, we’re seeing how other Saiyan choose to go about their business outside of battle.