Why don't rugby players have last names on their jerseys?

   Why don't rugby players have last names on their jerseys?

In the amateur game, players are assigned a position, and the jersey number assigned to it. No club (high school up to senior clubs) can afford to print up jerseys with numbers and names on them, and keep using those jerseys for years.
So, no names; you are just handed the ‘10’ jersey, for instance, before the game.

In the professional ranks (Premiership, 7s) players are hired for a specific position, so it is known that when a person is playing they will be wearing a specific number. If you pay medium bucks for a flyhalf, you won’t have him playing fullback.

Also, in the professional ranks, they don’t reuse jerseys - they just get ripped to shreds. So a team will buy a stack of jerseys in a certain size for a specific player who is hired to play a specific position. Chris Wyles of Saracens preferred to play wing, so there were plenty of jerseys with his name and number (11) on it. When he played fullback his name was not on the 15 jersey.

In Sevens the numbers aren’t tied to the position as closely, but you would still see Perry Baker with his name on his 11 jersey, supporting his nickname ‘Speedstick’ from his ‘11’ and his long legs.