If it was a C library... well, you couldn't really take advantage of all of the advantages C++ has vs C especially when implementing a windowing UI / application framework - inheritance, polymorphism, etc. really make a difference. If you did that in raw C you'd have, well - pretty much what we had to use for programming to the Win16 (now Win32) API before Borland's OWL (Object Windows Library) made the scene (this is before MS ever came up with MFC) - opaque handles to this and that, breaking down and handling Windows messages, etc. It was very low-level stuff, tedious and prone to error.
If you're happy with a CURSES-like library, that's fine (I've done my share of that also with C on DEC platforms, back in the VT-100 days) - but for anything this side of Y2K, application frameworks and OOP are the way to go. wxWidgets is definitely a valid way to get a cross-platform (and cross-language... look at wxPython) GUI app out there and still keep what's left of your hair.
Disclaimer: I've been using wxWidgets (was wxWindows) creating apps for a Fortune 500 international since the early 2.0 days (mid-90's).