They have shown plenty of scenes with Goh taking great care of all of his Pokémon and their needs across the series- frankly, more so than we've ever got from any other main character in the series, up to and including Ash.
The only time he's treated a Pokémon poorly is when he was giving Scorbunny a hard time just before it evolved, but he paid a price for that, insofar as him and Raboot didn't have a good relationship for a substantial number of episodes as a direct result.
I disagree, almost any other main character, aside from specific cases, has been shown caring and hanging out with their Pokémon constantly. Goh has had some few scenes with all his things on Sakuragi's Lab mostly limited to feeding them but we're left to assume he spends time with them. I think that's the problem, again.
I remember for example, that fans were complaining when Piplup had tons of screentime compared to the other members of Hikari's party, yet every one of them were constantly on-screen, doing little or big things. Compare with Goh, which doesn't have any Pokémon with storylines, a bit of personality development or screentime aside from his ace. Goh's Pokémon are on the level of something like Mao's Shaymin, Jessie's Lickitung or Ash's BW team aside from the starters; probably even worse. With some exceptions, all of them are devoid of any character and just function as pretty props.
I mean of course Goh isn't doing bad things to his Pokémon, but they are just there. Most of them are relegated to random appearances on scenes that depict the lab, just as background Pokémon. Raichu had its first appearance since its capture episode this Friday, same for Alolan Exeggutor. The little Magikarp I think cameo'd recently, but it wasn't seen before. Aerodactyl appeared 5 seconds to react to the new fossil-mons and that's all it has done. Have Scatterbug or Fletchling or most of Kanto bugs do anything at all? Wngull and Hitmonchan have had like 10 seconds of screentime total.
I mean, it's part of how the writers can't balance screentime for characters (in some cases it's almost borderline impossible) but it just highlights how little we've seen of Goh's Pokémon. They can't even do well with Ash's Pokémon right now, which are only 7 characters. Goh has 50+ things stuck on the lab, happily living there, that's a given, but they don't do anything and it creates the feeling that Goh doesn't care about them (like how Greedent wins the eating contest but it's not allowed to come to Unova to enjoy the prize).
In the end, it's all due to how the writers are "developing" Goh's goal and how different it is from the world already established.
