I had a nose job. Why can't I go back to doing judo right away?

Nasal healing. It typically takes 4-6 weeks for the nasal bone and cartilage to heal after nose surgery. Even minor trauma during that time could easily displace the nasal bones or cartilages. This could lead to a crooked nose or impaired nasal breathing or bleeding. You should avoid all contact ports during that time although non-contact exercise can be resumed in 7-10 days after surgery.
Risky business. A nose job is a surgical procedure intended to shape, straighten and support the nose. The operation often requires manipulation of the nasal septum, the tip cartilages, the middle vault and the nasal bones. While rhinoplasty can ultimately make the nose stronger, the nose is invariably weaker for 6-12 weeks after the procedure. Contact sports risk accidental fracture and shifting of the new nose.
Healing. For the first several weeks after rhinoplasty vigorous exercise should be avoided to prevent a possible nose bleed. It takes about 6 weeks for the nasal bones to heal together - so if you were to get hit in the nose before the 6 week recovery period the nasal bones could shift much more easily.
Healing time. If you get hit in the nose you will destroy the work done. If you exercise too strenuously you will get a nosebleed.
Nose needs to heal. Rhinoplasty involves the reshaping of the bones and cartilage of the nose. Unlike a building which is strong once it is finished, the nasal structure is weakest just after surgery. The body needs time to heal...Eventually the nose will be as resistant to injury as it was before. Particularly for a high impact sport like judo, i would strongly recommend adhering to your surgeons recommendations.
Nose Job Recovery. Your body needs to heal from the surgery. It takes a minimum of 6-8 wks for the scar tissue to become strong enough to withstand some force. Initially, you are quite literally being held together with tiny stitches which would not hold up very well to rough treatment. You need to let your nose have a couple months of "quiet time" to let everything become stronger before resuming risky activities.