Generally, nicknames are given to players that make so many plays throughout a game that announcers just need something else to call them by so the broadcast doesn’t get repetitive. Sometimes it works out, other times it doesn’t. “Greek Freak” is a perfect example of when it worked out.
At the time, most people didn’t want to take the time/energy to correctly pronounce “Giannis Antetokounmpo” and the nickname refers to his rich Greek background as well as his freakish body metrics. However, then you have nicknames like “Flash” that really just don’t age well. I understand that at the time it was given, Dwyane Wade was this explosive player that could rise above the rim with ease.