possible reason there might be to not wear dark pants, white shirt, jacket and a hat once one has left a Torah learning institution.
Practicality.
Even if there is no set uniform, certain jobs are done far easier without the garb mentioned above. Plumbers, appliance repair, bicycle messenger, lifeguard, mason, carpenter, veterinarian, etc. all come to mind.
Then, there is also the issue of “office culture.” A person may not want to “stick out” in the office by wearing a white shirt, tie and slacks every day if the office culture dictates casual attire.
The Wolf