Originally posted by: PSJ
Because they won't bother taking air time showing the same seals again, often they just yell the name of the jutsu when in reality(Naruto world) all seals are required to do a jutsu.
Rasengan don't require any hand seals, that also my theory why Sharigan user can't copy it. Hand seals are the identification of the Jutsus, and almost every jutsu we see in Naruto requires hand seals. That is probably why they said Sharigan can copy all jutsu's. Meaning Hand Seals, because Sharigan remembers everything witness,Unless Sharigan is psyhic and can read what the opponent is thinking, but Kakashi explained that those were jus false rumors. Jutsu's like Rasengan is all on the head, nothing to do with hand seals, I think that is pretty clever by the 4th Hokage.