Honestly, I think that this says a lot about how people think of NBA stars: it's not big men stars that are generally seen to matter in championship contention today, but instead big guards and forwards who initiate the offense from the perimeter (obviously Jokic runs a lot of the offense from the elbow and further out for the Nuggets, but there's a clear difference in the way he does that from how, say, Harden or LeBron runs an offense or how Kawhi ran the offense for Toronto in the playoffs). And, importantly here, the other major consideration is how well-equipped teams are to stop those types of players, both with good individual wing defenders and with switchable, like-for-like lineups.
That's the big question-mark for Denver, I think: they don't have a star guard or forward at this point (though they placed a pretty big bet on Murray becoming that guy this summer), and, if they run up against the Clippers in the playoffs, who do they have who can effectively guard Paul George over a seven-game series, much less George AND Leonard? Some mix of Torrey Craig, Will Barton, and Jerami Grant? Gary Harris or Beasley if they play up a position? Jarred Vanderbilt if he carves out a role? Grant and Craig are tough, Grant particularly, but you can see where the pundits' doubts are coming from there.
Utah added some quality wing pieces over the summer (Bogdanovic is an underrated defender and Jeff Green, for all the hate he gets, is still pretty good), but their big guards/wings who are back (Ingles, in particular, but also Royce O'Neale and Donovan Mitchell, despite his reasonable counting stats) all were a mess in the playoffs last year, both offensively and, more concerning, defensively. Conley should help take a LOT of pressure off of that group, which can give some confidence back in the offense, but Harden just completely carved up the non-switching, funnel-him-to-Gobert defense that Utah tried to stick with in the playoffs last year. Might just have been a particularly bad playoff matchup, but for a team whose star is a DPOY, their complete defensive ineptitude and lack of anybody who could even pretend to guard James Harden, one of those big guard stars who initiates offense from the perimeter, effectively (even Ricky Rubio is gone this year) is, to me, the clear reason why there's such a lack of faith in them. Who do they have to go to when they are forced to break out of their system offensively in the playoffs? And if their defensive system isn't working, do they have guys who can guard a Harden or a LeBron or a Leonard going iso play after play down the stretch of a playoff game?
Long story short, I just don't think that people see even the very best pure bigs in the game as gamechanging, particularly in the playoffs, in quite the same way that they do guys like Kawhi, George, LeBron, Curry, Harden, etc. And the fact that both Utah and Denver are pretty weak guarding those types of perimeter stars just compounds the issue, especially since neither Gobert or Jokic are going to be effective switching out onto those guys. Of course, this could all just be a hangover from the Golden State/Cleveland era of dominance, but that's my sense of the general perception these days.