Neither am I - hence FreeCAD.

But a very large number of users simply don't care about avoiding cloud solutions or using Linux. And OnShape does offer some unique solutions to low powered computing/agnostic OS uses and for the more corporate minded needs, the ability of two users to be able to work on the same model at the same time and see each other's changes in real time. Which can lead to faster collaborations and iterations of a design. And as someone who has wasted weeks of my life emailing design changes back and forth, this would have been a very handy thing at times.