Is there a reason why

DefaultSwitchColors

(and similar colour interface implementations in the Material library) don’t use `data class`es and instead manually implement equals/hashcode?

Yeah, what Harsh said! Jake wrote a good article about it: https://jakewharton.com/public-api-challenges-in-kotlin/

But

DefaultSwitchColors

isn’t a part of the public API, it’s private

Yep, the binary compatibility reasons are important for public API. In this case, using data classes would just add a bunch of methods to the published libraries that we don't need, so we might as well save some method count and bytes if we can.