Is this even true? To my eyes Japanese have many vastly different art styles while western cartoons have maybe 2.

Are you sure it's not just familiarity and contact allowing you to better differentiate different styles?

Staying within the 20 year rule and only picking examples from before 1996, I dispute your premise.

Does this:

https://gargarstegosaurus.files.wordpress.com/2015/08/logh-6b.jpg

Look like this:

https://i.ytimg.com/vi/WXSkaSkVBYA/maxresdefault.jpg

Or this:

http://vignette1.wikia.nocookie.net/dragonball/images/e/e4/MifanSoldierRoshiKuririnMysticalAdventure.png/revision/latest?cb=20120606222251

Or this:

https://i.ytimg.com/vi/PjrKKnwltZo/hqdefault.jpg

Look at the last two. Look how rounded Toriyama's characters are in Dragonball vs how angular and pointy the characters in Utena are. The variation in styles between those four examples (Legend of the Galactic Heroes [1988-2000]), (Mobile Suit Gundam: Char's Counterattack [1988]), (Dragonball [1986-1989, followed by Dragonball Z until 1996]), and (Revolutionary Girl Utena [1996-1997]) is vast.