Poor writing. The stories just weren't very good, the character writing wasn't very good, the events didn't have a natural flow of any kind. It also doesn't help to have a fairly weakly written villain.
I don't think the actors they chose fit the roles as they were written. Individually, all of the actors cast were fine. But watch any badly written movie, even the greatest of actors can't overcome terrible character writing. I also didn't see a lot of chemistry between the characters - the fault for this falls on everyone involved, those in casting, writing, directing, and acting.
Homecoming, on the other hand, checked off all of those boxes. The story is stronger (though still has some holes, as is typical of a superhero story). Character writing and casting has been a strong suit of the MCU, and Homecoming is no exception. The sequencing of things mostly makes sense. The characters have definite chemistry when they are onscreen together. And just in case all of that didn't work out, they went ahead and cranked the knob to 11 with their villain writing and casting.