One way or another, the Fantastic Four will eventually join the Marvel Cinematic Universe. Reed Richards already debuted via an alternate universe incarnation in Doctor Strange in the Multiverse of Madness, but fans have yet to see what a completely modern F4 movie will look like. While fans are certainly excited for Marvel's First Family to join the rest of their heroes on the big screen, perhaps they should temper their excitement and expectations with a bit of reality.
It seems that fans are more interested in the concept of the MCU being "complete" with the inclusion of the Fantastic Four than in the team itself. The same goes for the F4's villains, who are definitely of more interest in the wider Marvel Cinematic Universe. Add in the decades of irrelevance across several media, and it begs the question of why fans are so pumped for the four to hit the floor on the big screen.
The Fantastic Four Haven't Been Popular in Decades
While the team has had some good runs throughout their comic book history, most notably the original stories from Stan Lee and Jack Kirby, the Fantastic Four as a comic book property hasn't been at its hottest for decades. Even before they became as popular as they currently are, the Avengers easily eclipsed them as Marvel's premiere super-group. Then there's the matter of the X-Men, who, once they hit their stride with writer Chris Claremont, put both of Marvel's other teams to shame in terms of popularity and influence. Even with acclaimed modern runs, such as the one from Jonathan Hickman, the Fantastic Four comic book has never been the talk of the town with today's comics readers.
In terms of outside media, the view is even bleaker. Back when Spider-Man and the X-Men had hit cartoons in the 1990s, the Fantastic Four cartoon of this era was an utterly laughable affair that would have only convinced kids that the team was lame and outdated. The actually fairly underrated animated series Fantastic Four: World's Greatest Heroes was much better but sadly didn't get much traction. Of course, there's also the team's lack of luck in the movie department. Said films, namely Fantastic Four: Rise of the Silver Surfer and Fant4stic, have put a huge black mark on the property with the general public, who likely see them only as characters from numerous failed movies.
Of course, the MCU has turned characters and teams such as Ant-Man and the Guardians of the Galaxy into relative and huge hits, but the F4 is a bit different. The Marvel Cinematic Universe has never "rehabilitated" a property that was previously poorly adapted, let alone one with such a poor reputation. The general audience didn't know what to expect with Ant-Man or Guardians, whereas Fant4stic, in particular, is considered one of the worst superhero movies ever made.
Even Fans Only Care About the Fantastic Four's Villains
At the end of the day, a lot of hype surrounding the Fantastic Four officially joining the Marvel Cinematic Universe has to do with their villains. Doctor Doom is one of the most iconic and prevalent supervillains in the Marvel Universe, and his joining the MCU could also lead to the eventual Secret Wars adaptation. Likewise, Galactus is a huge door into the cosmic side of the MCU, and his looming threat could become the new equivalent to Thanos. Even though these characters would require the F4 to be introduced (and not anger fans by their absence), the villains are much bigger attention-grabbers to casual audiences than "that rock guy" or "the dude that can stretch."
While villains like Norman Osborn or Magneto are popular, no one would expect them to show up without Spider-Man or the X-Men. On the other hand, the Fantastic Four are almost unwanted narrative guests who have to be brought in for the more desirable Doom and Galactus to be introduced. This all stems from their relative lack of popularity compared to other big Marvel properties. Marvel Studios could definitely change that by finally giving the team a great movie, but the question then becomes whether audiences will give it a chance. On top of that, audiences seem to be over their love affair with the MCU, becoming far more critical and disinterested in its latest offerings. This puts the supposedly "huge" draw of an MCU Fantastic Four movie in perspective, and it'll definitely have more of an uphill battle than some fans think.