I did explain most of them, here are the rest. I feel this is kinda obvious but since you asked...
F-Spot: super slow, development has been stalled for years, modifies your originals (fixed now?), clear dead end.
Evolution: difficult to set up (especially calendaring!), mostly stalled development, ugly, buggy.
Amaya and FreeCAD need no explanation -- they simply don't work for any reasonable task. In a few years hopefully FreeCAD will have something.
Unfortunately, I don't think there's anything I can do to help these programs catch up. They're outside my circle of expertise and the hours in my day are limited.
The EE CAD programs you mentioned from college, were they open source?
Related: I used a TCL-based VLSI layout program on Linux to lay out a crazy fast SRAM chip in college (forget the name). It was mostly closed source alas but I was able to tweak some of the TCL to speed repetitive tasks. I got it working but it was a lot more painful and time consuming than the popular EDA packages at the time, especially when it came to simulation! My takeaway: for the right program in a commercial setting, $15,000/seat is downright cheap, especially if it saves man-months during a project. (keeping those horrible dongles happy though, that's too expensive at any price)
As for why mechanical CAD programs are only on Windows? I only have a guess: blame the Unix wars. They're graphically very intensive and difficult to port. There's just not enough money in Mac or Linux to justify the effort to bring them back to Unix.