The insane thing is, the 4e DMGs (both of them; can't speak to the DM's Kit, but I'd be shocked if it were different) does exactly this. It's all about improv, saying yes, and helping your players have a fun time.
It needed to be pushed on the player side more, I agree, but it's seriously all there to even casual perusal.
You kinda make my point by saying it's only in the DM book, which many of the players will not read.
Amusingly, I was just saying how bad the example of improvisation on page 42 was. It's meant to be an example of something rules do not cover, but then it goes and describes a Bull Rush. And overtly comments how underwhelming improvisation (according to the rules) compared to using a power.
I don't recall seeing much of this in the DMG 2. But it's been a while and I wasn't looking so I could be wrong.
Essentials did have more examples, but mostly tied to skills. And still a vast minority of the book.
Which is the thing, even in the DM books the advise was subsumed by combat. There was two pages on improvisation, and just advice on configurations of monster roles received more advice. The part of the game that required the most skill and DM adjudication received the littlest attention.
And while there was the three non-Monster or campaign DM products, there was how many player books?
And, in the book on being a good player, the book on running a character, how much talk was there on improvisation, how to improvise, when to improvise, etc.
Now, with that said, WotC's 4e adventures up until about the Essentials releases were pathetically bad, ignoring every good piece of advice the DMG suggested. I think it was a combination of the delve format, their general lack of knowledge about their own system, and page limits wherein they apparently needed to fit 3 levels of XP within 64 pages. Regardless, they shine a huge spotlight on 4e's worst flaws rather than celebrate the stuff it's actually good at.
The delve format was pretty bad. Even encounter had to be tactical because the format included maps and monsters. And had to be combat, or they'd have spent all that time formatting and working on the encounter for nothing.
It's pretty emblematic of 4e's problems.
I did a blog a while back on
why 4e was all combat, (cheekily named because a more apt name would have been "why, all other things being equal, 4e will slide towards combat") and one of the points was how encounters were designed.
Look at the 5e playtest: I've heard of numerous groups just charging overwhelming foes and getting slaughtered, because they've been trained by two editions to just assume an encounter is balanced and a fair fight. And if the fight is fair, there's no incentive to think of other solutions.
Very, very little of the combat centralism was purposeful design, but rather a series of accidents: synergies from individual improvements to that game that, taken as a whole, encouraged combat.
A good and skilled DM can sway away from this. And players used to improvisation and more narrative combat might not notice.