Because while default scroll is fine for scrolling pages whole, in any scrollable div that's not top level it looks out of place like a blacksmith on an iceberg.
It's a gray slab that looks like a tombstone riding a straight, dull rollercoaster and unless your site makes a point of having a '90s inspirational look there's no way to integrate them in a page so that it doesn't feel completely detached.
It's a gray slab that looks like a tombstone riding a straight, dull rollercoaster and unless your site makes a point of having a '90s inspirational look there's no way to integrate them in a page so that it doesn't feel completely detached.