That they "drilled down" I think represents part of the difference, they basically looked for the most serious aspect of the source material and then emphasized that very heavly and obviously onscreen in a deliberate attempt to make the work stand out as having weight.
For me, it's more like Singer's X-MEN, Lee's HULK, and Nolan's BATMAN were allowed to put their stylistic hallmarks on both character/story and visual style. Inasmuch, they were allowed wide latitude to create whatever film they find truest to their imprimatur. They were their own Feiges. I'm not sure they worked in a deliberate attempt to elevate comic book material, so much as that's just who they are.
Getting back to your original question, these filmmakers didn't have the technological freedom that the MCU presently has, so in order to create a respectable story that didn't read on screen as implausible -- or "camp" as I believe you're calling it -- they had to make changes to the story and/or the character. Here lies the argument of which Wolverine we'd rather see -- over six foot Hugh Jackman shredded to zero body fat, or a baby bear in a blue and yellow suit with epaulets?
As film methods advanced to the meet the challenges of depicting comic book spectacle, and as the film audience grew to accept them, there became less necessity to make creative changes to story/character. Just like the advent of digital film gave rise to more improvisational filmmaking.
Fidelity is easier to maintain. It's easier to churn out more and more impossible-looking scenarios.
With a wealth of story history the new adaptations need only make the barest updates. Ergo, the MCU is successful in the same way a really good cover song becomes your favorite version of that song. It's familiar, and updated BUT NOT UPDATED TOO MUCH, just in the way you like.
TLDR: the CGI isn't horseshit, so it doesn't look silly when a wizard fights a grape monster.
The Marvel films do include themes of depth but they tend to go out of their way much less to highlight them I'd say and are happier to include comedic and larger than life elements within them. I mean I think theres actually a lot added to them in terms of specifics of characterisation especially but they do I'd say feel like truer recreations of the general tone of the originals for me, not films with something to prove.
I don't know about "something to prove." I think that's a perspective of the outside looking in.
The MCU maintained the goldilocks path of refining a product. They didn't dance too far outside the lines stylistically and rarely extended themselves past present-day techniques. Overtime they gathered tremendous steam to create a great cinematic experience with INFINITY WAR, and more importantly they primed the audience to hit at just the right time to maximize impact (profits + product).
My major strike against the MCU is that their stories aren't about much more than rehashing the storylines. Yes, they've updated them nicely and tied disparate strings together onscreen, but there isn't much more resonance beyond the nostalgia and spectacle. I'm not sure they're adding so much to characterization -- I think that's just a byproduct of casting a living human being. As a result the story looks very pretty and it's very pleasant, but it's just a bit hollow and, to be honest, more akin to television than film.