When desk-top publishing first came in (remember that term?) it was easy to draw boxes with rounded corners, something that had previously been very difficult to achieve with Rotring pens, CS10 board and french curves (ask a designer over 40 to translate).
Because they were so hard to do, they weren't in the professionals' repertoire, and therefore a sign of an amateur at work.
In this guide to design for DTP commissioned by Monotype in 1991, rounded corner boxes are a feature of this amateur-hour how-not-to example.
I've changed my mind now, although I'm still not keen on long line lengths, floaty headings in upper case, and needless naff shadows.