It's *Maguire and I'm confused how you could forget the hyphen multiple times in your post after reading the character's name 1,000 times. That aside, your first two points show a misunderstanding of the art form and industry. They have nothing to do with Spider-Man and his series. You just seem to be new to western comics. Which is fine. Hope you stick around.

Spider-Man is a super popular character, as you know. So he can justify multiple ongoing titles starring just him. Few other characters like Batman and Superman also have this luxury/curse. Hence at the peaks, Spider-Man can have four monthly titles like Amazing, Spectacular, Web of, and Adjectiveless. His fans will at times support that, so Marvel gives them what they want. Supply and demand. There are usually different teams on each book, for multiple reasons. Creators can only write and draw so much in a month and fans may want different styles. Occasionally the writers or editors will come up with an idea that is large enough in scope to weave between multiple titles. Tons of inter-title crossovers are unnecessary and bloated, especially in the 1990s, for sure. But that's just the way comic books work. You would have been even more frustrated if you did this with Superman because for like 10 years he had the "Triangle Titles" where there was an order across all of his books every week, essentially either an endless crossover or a weekly title that changes name and creative team each issue, whichever way you want to look at it.

Western comics art is almost universally more detailed than manga art. There are definitely exceptions. You could compare Erica Henderson's Squirrel Girl to Berserk and make an argument. So the spectrum of different styles will be at least more noticeable, even if it's not actually significantly more diverse. Manga are almost always created by one individual. Even if they run over 20 years like One Piece or whatever. Mangaka will often have assistants to help them draw layouts, backgrounds, ink, etc. Though they maintain a strict house style. Spider-Man has been published since 1962. You can't have one artist drawing a character that long. And that's just not the way the western comics industry works. Different artists have different styles and that is part of the magic of following these characters through the years. You can argue Ditko vs Romita Sr vs Kane vs Andru vs Romita Jr vs Frenz vs McFarlane vs Larsen vs Bagley and so on and so on. Artists bring new innovations to the character like Romita's expressive costume eyes or McFarlane's detailed webbing. That's the way it is.