Djokovic won his first Grand Slam in 2008 and he has been proving over and over again for 17 years that he's the greatest tennis player of all time. That must still drive some people crazy. Under normal circumstances a player his age should've retired long ago, yet here he is reaching his 19th Wimbledon semi final. You don't even need to be a tennis expert, anyone with the slightest knowledge of the sport knows how important Djokovic's place is in the history of tennis.
The term GOAT is extremely subjective and is thrown around across different fanclubs from literally every sport on earth. Djokovic is one of the greatest players to play Tennis for sure, but he is just one among many.
Federer, Nadal, Graf, Court, Serena Williams, Laver, Sampras etc are all some of the greatest players to grace this sport too.