I think you're right that that has led to people's thinking of rectangles as 'default' (and because they're more traditional, probably for the same reason)—but

I think you're right that that has led to people's thinking of rectangles as 'default' (and because they're more traditional, probably for the same reason)—but it's an irrelevant fact now. However you're drawing button these days, it's generally a choice between drawRect(...) or drawRoundRect(...); or of course in CSS you just add one line to define the border-radius.