I don't mean to be combative, but I just finished watching an 26-episode action anime where there was a gunfight in almost every episode (I'm pretty sure it was all of them, but I could be wrong).
Most of the action anime I watch are like that. There's occasional downtime, but you always have this sense of looking forward to the next conflict, because that's the tone of the show.
If anything, I would argue that long-form shonen anime are a different category of "action" entirely: having a serious battle in nearly every episode of a 100+ ep series (that's actually supposed to be story-driven) would be both a strain on budget and logic; it just can't work the same way. Longer stories need longer spaces between high-energy moments.
Anyway, to actually answer the topical question, since I forgot to do that before...I think @darkone4587 summed it up pretty well. That and probably a bit of good old-fashioned fear of change.
After all, the energetic-slice-of-life formula works so well: it's simple to write, and you can drag it out for years if it does well, unlike a serious series that will probably demand an ending at some point. So why make waves; why take a chance? Why greenlight the next Avatar TLA when you can get Teen Titans Go 2.0 for half the price per season? Even if it bombs, at least you didn't make a big investment. =/
I think that's the reason indie media evolves so much faster than traditional media: the latter is hampered by people who just want to make money, and are thus reluctant to try new things. So once they find something that works, they stick to it, everyone copies them, and for a couple years basically nothing else gets made.