Even in airlines it is now becoming compulsory for candidates to wear uniforms for some or all of their simulator details. In the past it was rare to find such a requirement, since that type of training was outside of the public eye. However, there is a strong body of opinion that a pilots performance is likely to be enhanced If the environment is made as realistic as the situation allows, and to that end wearing uniform is a part of that realism.
Professional pilot flight schools gear up much of their day to day operations to prepare the candidates for an airline career. Their operations rooms, teaching facilities, flight planning, line operations, etc. are all designed to replicate as best as possible those likely to be found upon successful graduation. Obviously wearing a uniform is also an intrinsic part of that operation.
Had I won the lottery last night my soon to be appointed chauffer, butler and GV crew, would also have been bedecked in a smart new uniform. Even the bodyguard would have a smart suit.
If somebody wants to jump out of their C152 in a uniform of their own making, why should I care?