I belief there are many reasons why round is accepted as normal.
Pocket watches from centuries ago.
The dial of a clock.
Sun hand.
Practicality:
Better water resistance.
No sharp edges that can pring in skin or in a pocket of a pair of trousers.
Round is referred to coins that said you are rich.
Rectangular is as i see it, a fashion related subject.
Rectangular is odd and show a more individual view of an owner, a fresh thinker. Somebody who can push boundaries.