Why do so many of Brazil's soccer players...

No, thats nicknames or they only use the first name because they have a long name, for exanple:

Fred: Frederico Chaves Guedes

Kaká: Ricardo Izecson dos Santos Leite

Ronaldinho: Ronaldo de Assis Moreira

Ronaldo: Ronaldo Luiz Nazário de Lima

Dida: Nelson Jesus da Silva

Juninho: Antônio Augusto Ribeiro Reis Júnior

 

cause they're so ****ing good, that's why.

 

ya learn somethin new everyday, thanks.

 

but they should use their last name or the shorten version of it, like Mutombo (i forgot Deke's full name )

 

His first name is pronounced "Duyuwantusex"

 

I thought they legally changed their names, like Nene.

 

I think david_rocket is right. The players get to choose what is on the back of their Jersey.

for example, the dutch player Giovanni van Bronckhorst has "Gio"on his jersey with Barca. But his legal name is still Giovanni van Bronckhorst.

 

That's the Brazilian convention. Nicknames and first names are used in all settings, no matter the gravity. Brazil's president, Luiz Inacio Lula da Silva, is known to all by his nickname, Lula. Clergymen, doctors, and other professionals are frequently known by an informal name. The phone book for the town of Claudio even lists inhabitants by their nicknames rather than their surnames.

Brazil's affinity for nicknames might stem from the country's historically high illiteracy rate. As such, shortened spoken names are typically used more often than longer birth names. In Brazilian society, the use of a first name or nickname is a mark of intimacy. It's also often a class signifier. Lula, for one, is known for his working-class roots.

http://www.slate.com/id/2143404/