Finding a doctor, making an appointment, keeping that appointment, trying who knows how many medications until you find one that helps, etc. is not the easiest thing in the world when you have trouble just getting out of bed in the morning.
It wasn't until a few years later that serialized TV even really became a thing -- Twin Peaks probably was the first here, but Babylon 5 would have a good claim (and DS9, Buffy, and others were coming together then too).
Soap operas were doing serialized storytelling for decades before your examples. Maybe not good serialized storytelling, but still.
Most of the TOS cast was on board, I think only Walter Koenig didn't participate. As for other voices, most of those were done by TOS castmembers with James Doohan in particular having several non-Scotty roles.
Finding a doctor, making an appointment, keeping that appointment, trying who knows how many medications until you find one that helps, etc. is not the easiest thing in the world when you have trouble just getting out of bed in the morning.