Why not use Linux with WINE and that Chicago95 theme and call it a day?

That's (part of) my point. A project like ReactOS which clones Windows down to the kernel level solves for a very small set of practical use cases which are not covered by real Windows, or Linux+WINE.

It's worth noting that 30 years ago, there was a definite advantage to an open source operating system which could reuse proprietary Windows drivers - even Linux had a mechanism for using Windows drivers for certain types of hardware. Nowadays, Linux provides excellent support for modern PC hardware with little to no tinkering required in most cases. I have seen many cases where Linux provided full support out-of-the-box for a computer, whereas Windows required drivers to be downloaded and installed.