He's not
that bad, he tells actual stories, kinda, he's just the laziest plotter in the history of script writing.
"They throw Kirk off the Enterprise! And... uh... Old Spock just happens to be there! Like, right there! Because it's less work for me if he just is, for no reason! I don't want to think of a real reason and maybe have to go back and revise stuff, or remember things later, to integrate that! He just is!"
"They escape on the Millennium Falcon, and Han and Chewie show up... uh, I guess they'll need another ship for that, so now we have two. And I guess there was a homing beacon on the Falcon, that got activated, and Han just happened to be about five minutes away and already sitting in his cockpit and ready to go and not already on the way somewhere else, in hyperspace? Yeah, doesn't matter, I just need him to be there now so he is, doing something else would be hard. Oh shit I have two ships now, and I need them all on the Falcon, so, uh... monsters and pirates destroy the extra ship I just introduced! Yeah there are monsters and pirates now. Do either of those stick around or come back, or are they, like, foreshadowed? No, I just needed them to destroy this ship that I manifested for my own convenience, but no longer have any use for."
"I need these characters to go somewhere else, so, uh... someone/something shows up and chases them away! There are no other consequences or meaningful connections, they just get chased so that they leave and go to where I need them to be."
Et c.
[EDIT] To be fair, most writers do some of this, they just don't do it quite so willy-nilly and build entire plots that way.
[EDIT EDIT] This also doesn't necessarily mean his movies are bad. The plotting... I mean, that part's bad, incredibly bad, like, paragon of bad plotting, should-be-covered-in-film-schools bad, in some of them. But it's not impossible that a movie could still be entertaining, or even good, despite that. I don't mean this to dismiss his entire body of work as absolutely and entirely bad, and actually started out defending him as markedly better at everything than the kind of scripts and TV shows I was writing about, which he certainly is.