I can buy AT-to-PS/2 converters; some might even work
correctly. Why
would I want to build one?
Because 'some might even work correctly', implying that some do not. If
you deisng it yourself, it's going to work correctly. And it's going to
be built correctly.
That's quite apart from the education you get designign and building
soemthing like that (OK, you know how to do it, there are plenty who
would learn by doing soemthing like that).
-tony