[SwellJoe]

Some of them were. And, UI is a hard problem that rarely gets a lot of attention in Open Source. For many reasons, designers very rarely donate time and effort to helping out OSS projects, while coders much more often do...so, even if the technical side of an OSS project is solid and reasonably modern, the UI may trail many years behind.

It takes a few years to build a solid mail client with volunteer labor (or even with paid labor; Mailpile, while promising, is nowhere near ready for production use, for example), so none of the full-featured, stable, ones you'll find are newer than five+ years old, and many are much older, and often the designs were made as an afterthought by the developer rather than someone focused on design.

Open Source has always had a problem with UI quality. That's not new, or specific to webmail. I don't know the solution (the webmail I work on, Usermin, is getting its first UI overhaul in about a decade, as we speak, and only because a good UI person stepped up and started working on it on a volunteer basis, and then we started paying him).