Priests have the option of wearing a Cassock if they choose to do so, and if their Bishop allows the use of them. Religious Priests often wear a Cassock, if their community has one. A Franciscanās robes are just a form of a cassock, as an example.
Do you wear knee breeches, and a tri-cornered hat? After all, that was the customary garment when the US was founded (in Europe as well as the Americaās), so why not wear it today?
Just as lay clothing has fashions, so does religious clothing. Many of the Communities of Nuns had Habits that were based upon the common clothing worn at the time they were founded. But, just as lay people do not wear the dress of the middle ages, why on earth do we expect Priests, Nuns and other religious to do so?
They should however, wear clothing, or some form of attachment, that clearly identifies them as religious. To just wear lay clothing, unless āoff dutyā, is not allowed.