In the book on which the movie is based, it says:
Logging out of your OASIS account while you were engaged in combat was
the same thing as committing suicide. During the log-out sequence,
your avatar froze in place for sixty seconds, during which time you
were totally defenseless and susceptible to attack. The log-out
sequence was designed this way to prevent avatars from using it as an
easy way to escape a fight. You had to stand your ground or retreat
to a safe location before you could log out.
I assume that the immediate log out shown in the movie was only available if you weren't engaged in PVP combat.