Now that Marvel's built a mighty film empire of movies starring rogueish-but-good-hearted superdudes for itself, it's hard to imagine that their first feature film EVER came out less than 30 years ago. It's even harder to imagine that Marvel's first theatrical film was about Marty McFly's mom trying to bone a duck.
Prior to Howard the Duck (released in 1986), the last piece of Marvel anything to be shown in movie theaters was a Captain America serial back in the 1940's, back when movie serials were a thing (and when Captain America was pre-frozen). The real crazy thing? The next Marvel film to see a theatrical release wasn't until 12 years later - 1998's Blade*.
* - although in-between they released several direct-to-video low-budget versions of The Punisher, Captain America, and - infamously - The Fantastic Four.
The natural place for most comic book movies should logically be animation - comics themselves are drawn, so that they can include absurd, insane action and creatures without breaking the bank. Plus, some drawn things would just look too goofy in real life - such as an angry humanoid duck, or another humanoid duck who has duck-boobs.
That isn't to say there isn't a lot of value in giving a comic book a real life adaptation - the Batman films, Iron Man, and a million others have been ridiculously and rightfully successful. Even Guardians of the Galaxy - a film that prominently features an angry humanoid raccoon - has been a huge success. The problem Howard the Duck had in going live action was twofold - one, James Gunn didn't write and direct it (he was busy being 16 years old), and two, Hollywood's special effects weren't sophisticated enough for Howard to not look ridiculous.
The weird thing is that they knew this - and had originally wanted Howard the Duck to be an animated feature. Writers Gloria Katz and William Huyck had written the film intending it to be animated, but then the film was optioned by Universal Studios. Universal had previously passed on a few wildly successful things being produced by George Lucas (minor films like Star Wars and Indiana Jones) and were gripped by a bad case of FOMO - but they also needed a bigtime summer tentpole release, and an animated film wouldn't cut it. So they decided to turn Howard the Duck from a surreal satirical animated film into a live action summer blockbuster.