Traditionally
GTs were
GW's event and
GW wanted to promote all aspects of Teh HHHobby, which obviously includes painting. Thus a fully painted requirement was standard at
GTs.
Why is painting a standard aspect of war games? The fact is that the majority of people who are interested in war games are interested in nice looking painted armies because nice looking painted armies have been a key attraction point of war games since ancient Egypt, as shown by these model soldiers from an ancient tomb.
It never mattered whether you painted the figures yourself, or bought ready painted figures. The point is that painted figures look nicer and that is what attracts people to play with them.
People who only want to play the rules, and aren't interested in painted armies, and play with unpainted armies, are a minority. They cannot reasonably expect the majority, who don't want to play with unpainted armies, to want to play with them.