Why does nobody train his pokemon?

I mean, why are they even called 'trainers' if they never do
what the title says? The only time in the series i saw
Ash actually train was at a Johto episode, where he
trained his Cyndaquil to light the fire on its back faster.
Ok, i know Ash is lazy and wins his matches mainly
because the plot requires that he does. But what about
other trainers? Brock and Misty are gym leaders, they're
supposed to be serious trainers. Yet, with the exception
of the times they take them out of the pokeballs for
battle, they never seem to bother with their pokemon.

Because they train off screen

The pokemon train themselves when they are playing with each other. And they always train in every season or imply they they do. I haven't seen a saga in which they don't have at least a few training episodes.

Sun and Moon has had training episodes with multiple characters. Unova didn't have as many, but Ash and Iris both battles one another on and off in episodes or were battling while the episode started.

Ash trained with all of his pokemon in Sinnoh. He trained with all of his pokemon in Kalos. Unova he battled and trained with Scraggy. There was so much battling in Unova that you really didn't need training episodes. Alola he has trained with Lycanroc, Litten, and Pikachu. Hoenn he trained with Sceptile, Swellow, and Glalie.

He has trained enough for an anime. You can't have an episode with just training or it becomes boring. Usually when he trains, it is to help his pokemon learn new moves.

All other trainers train as well, but they just don't train for battling other than Iris and Misty who constantly battled Ash for training. May, Dawn and Serena were all in fashion/contest things, so the trained differently, but they still trained. Clemont was a pretty strong gym leader so he didn't have to train as much. Although he left the main cast for a few episodes to do just that. Brock is a Pokemon Doctor and Breeder, if I am not mistaken, in the anime he never really wanted to be a Gym Leader, he wanted to travel. Tracey was a researcher, so he didn't train as much.

Cilan is the only traveling companion that lacked any real training, but he still battled constantly with Iris and Ash.

In Alola Kiawe, Lana, and even Lillie have trained some. Sophocles has no real interest in battling so he doesn't train. And Mallow just hasn't had much character development, but she is more of a cook and basically a model so she doesn't train.

Who needs training when you have spontaneous out of nowhere BS crap to rely on

Because OS was bad.

OS did focus more at highlighting average regional Pokemon per episode than letting Ash or Misty train. I find it okay to give spotlight to the focused Pokemon of every single episode but they abused it a way too much. AG and DP reduced the spotlight of average fosuced Pokemon per episode and they focused more on training.