Because buttons weren't distinguished by color in Windows 9x, but by borders and shadows. You can do that on the web too, but this article is about color.

You can make a usable product using a Windows 95 palette, but I think it may look outdated to users.

Depending on your target market, the color scheme you use can have an effect on how your product is perceived by your potential customers. User tastes and expectations have become more sophisticated since the 90s and users expect a higher level of polish now than back then. That can include color choice as a part of overall branding.

It must be possible to achieve a happy medium: something with the legibility of an older design that doesn't look dated or unpolished. (Though I reject the notion that 90's designs weren't sophisticated or polished.) It doesn't need to limit itself to six colours, but I'm not convinced that an endless series of shades with low contrast to one another is good design from a functional perspective.