Why do you say do judo not play judo?
You say "do judo" because it isn't usually casual, and doesnt
require a team, just two people. Such as you play football, but you
do Karate. The term of "i do" is usually refered to any martial art
that is practised, rather then "i play". This could be related to
the feeling that to do something is to work and improve, while play
is more casual.
While this is true, I disagree. Judo is a sport, a most well
known sport at that. And much as you would say play football, you
are playing judo as well. Yes, playing is casual, BUT it is used to
describe a sport or game, such as judo. Using the term play just
like you would say that you play chess, not do it. While you play
judo though, it would be like playing in a baseball game, while you
would practice to work and improve your skills. Just like at a
tournament you would play judo, at a class or in a Dojo you would
practice it.