Sharp corners in RFID antennas
RFID "antennas" are really just induction coils (unless you're talking about UHF tags).

As such, their performance is almost entirely determined by the area inside the loop, and a rectangular loop is the best way to maximize the area if you're constrained to a rectangular tag.

The rounded corners are simply to prevent excessive concentration of stress in the wire during the manufacturing process.

It is true that in the absence of any other constraints, a circular loop will give you the maximum area for a given length of wire, but this aspect is usually overridden by unrelated requirements.