Because it takes way too long, this is faster, the issue is people don't want one pic and move on. They want a pic with them, then one with Grandma, then one with Grandpa, then one with everyone, then one with two kids, then one with each kid. Then they decide they also want an autograph and to give everyone a hug. It just takes FOREVER, there are too many people.
this way you get a snap and move on.
You have to remember that in Asian parks people will seriously line up for over 8 hours and longer for a ride, parade or character. They can't have characters creating these huge lines that people WILL wait in, even if they had more characters it would just create more huge lines. You're underestimating how long people from Asian countries will wait.