https://en.m.wikipedia.org/wiki/Star_vehicle
Who cares? It was fairly entertaining as long as you find Chevy Chase funny.

Considering that CC is in every single scene in the movie I guess you can put it that way if you like. But there's hardly a bad scene in the entire film. Sh-t's entertaining as hell - and pure 1980's through and through.
"facts are stupid things" Ronald Reagan

The ENTIRE movie is the reason to watch this movie.

The ONLY reason to watch this is Dana Wheeler-Nicholson's manjaw
.
. Ephemeron.

There are others.

• The premise (relatively rare)
• Atmosphere (Pure '80s at its best!)
• Incredibly wonderful, energetic, atmospheric soundtrack with that 1980s 'vibe'
• Relatively unique and unpredictable storyline (although the end is eventually predictable, but for a long time, it feels very unique, not done many times in other movies quite this way, you never know what's going to happen next or what to expect, and it even has some twists)
• Dana Wheeler-Nicholson (she was a bit of eyecandy in this movie, despite her odd eyelids and manjaw, almost nill charm and bored, lackluster performance - it all somehow fit this movie's style very well, so it's all good)
• Interesting chase scenes
• Really funny comedy moments (which this movie is full of)
• The gorgeous mansion (could be just me, I just like mansions)
• The sceneries
• The arrogant boldness of the character (what he does with Mr. Underhill..)
• This movie is actually great, delightful fun, while still feeling..
• ..somewhat intriquing and seriously interesting plotwise (which is rare in comedies)
  I could go on, but I think I made my point; Chevy Chase may be A reason, or even THE reason to watch this, but he's not the ONLY reason.

I can think of hundreds of films that would be forgettable had it not been for the talent and acting of the star. Is this supposed to be a big revelation or something?

but the story was really poor and even though they did managed to tie all the threads together by the end, it needed so much exposition.
The plot jumped around so much, from the beach to the country club to the office to a trip to Utah, back to the beach, then the police station, back to the country club, another trip to Utah blah blah blah, by the time it came to confront the character who had taken the hit out on himself I'd forgotten that that was even part of the story!
Your description could apply to plenty of other mystery films, such as Chinatown, Night Moves and The Long Goodbye, or even conspiracy thrillers like the Parralax View and Three Days of the Condor. Fletch is actually more forgiving on the viewer, offering the occasional narration to help the viewer along. None of the movies I mentioned have any such thing, and even less exposition than Fletch.
The closest thing I can liken it to as a crime-comedy vehicle for a comic actor is Quick Change with Bill Murray (and coincidentally Geena Davis again), this manages to accommodate a show stealing performance from its lead and be a coherent and interesting story at the same time.
Fletch isn't incoherent by any stretch of the imagination. Certainly it begs for repeat viewings, but then so should any decent mystery story.
"It's just you and me now, sport"

Manhunter