You do, but companies don't. A major advantage of a browser app over a native app is having one system that behaves relatively consistently across all platforms. This isn't just cosmetic: providing customer support for a web app that has exactly one expected set of behaviors is far easier than asking your support agents to keep track of the constellation of different features users might have access to depending on their platform.

The color picker control is the antithesis of this consistency: not only does it look different across platforms (making it harder for CS to walk customers through something over the phone), it provides different features depending on the platform. Some users will be able to pick an arbitrary hex code, some will be able to pick colors off the page, and still others will be given a tiny set of fixed choices.

HN's audience might prefer that because they know the platforms they use extremely well, but the average caller to a customer support desk doesn't know their own device that well. If we instead write a bespoke control, you (being technical) will be able to figure it out, and the people who call in to customer support for help will all be seeing essentially the same thing.

EDIT: And note that when I say "companies don't", I'm not even just talking about the SaaS vendor. Oftentimes the customer is also a business that has an internal support desk that prefers consistency.