Because audio is easier than video.
I think when it comes to endlessly long series like that, the formula starts to show through the cracks. The challenges and obstacles may be slightly different but the structure will remain the same. This makes it easier to predict what will happen, so the story loses some of its mystery. I guess it's ironic, the audience becomes over powered.
Gosh, I hear you on Dragonball being more enjoyable. I never had much interest in DBZ either - I started with Dragonball and when Goku got older, the series did lose that luster. I too watched Sailor Moon but a lot of my anime stems from all of the stuff Adult Swim showed - Cowboy Bebop, Outlaw Star, Big O, Ghost In The Shell. I got tired of all the tournament stuff, it's just too easy to figure out. I can understand endless fighting for final battles but don't carry it out like Bleach does or have constant fight ups like Dragonball Z or Yu Yu Hakusho - though, the season after the Dark Tournament is actually really good.
I've noticed that with a bunch of series too.For me the the most recent example could be Bleach.Started watching it, completely interested in all the ghost fighting, missed a couple episodes, then when I came back to it it was about fighting and pretty much nothing else.Then again it could have just fallen prey to my lack of watch.But yes, never got into the Dragonball books and although I've seen most of the Z episodes and some of the original anime (thank you cartoon network)I just couldn't get crazy into it. When a fight takes over four or five episodes to get over I throw my hands up and opt out.Preferred watching Samurai Pizza cats and Sailor Moon...and yes I was also obsessed with her.Even opted to watch it in Spanish on a family trip even though I couldn't understand a word. Better than missing it.
Ah I know I'm late for this but I'm just going to go ahead and comment on it xDTo be honest the whole process of the plot devices for manga depend heavily on the viewers. I LOVED the original YGO (i have the VCDs somewhere stil!) but along with Naruto, Reborn, even DBZ they all turned into battle manga (or tournament themed manga) because had something, at least a little bit, to do with the ratings dropping each issue and what not. I'm not an expert on that but yeah Battle manga (or tournament themed manga) is just very well received with the younger audience ^^;;
After listening to this particular podcast, I know I have to recommend you 'One Piece'. It has been running for about 14 years and the adventure concept never changes. The main hero, Luffy and his crew just sail around doing-not-very-pirate-stuff like they did in the beginning. There was and never will be a tournament of any kind, there is a straight goal for the adventure, there are mysteries and a ever expanding universe. It will give a new perspective on shounen manga nowaday.