Why doesn’t Fonzi ever change his clothes?

Sacrifices required to maintain that level of cool.

So did Ben, Adam, Hoss, and Joseph Cartwright.

He probably owned multiple white t-shirts and jeans and never bought anything else.

Or maybe he was color-blind so he made sure he had only white shirts and jeans so his colors would not clash.

Standard practice for series television from a production standpoint. Giving a character one look makes editing easier to do saving money. Archie Bunker always wore a white button down shirt with brown slacks which I thought was pretty weird but they stuck with it unless Archie needed to dress formal for some occasion.