We know that this is not permitted if we are attributing negative qualities to a group of people. Why is it then permissible to generalize when we are praising the same groups?

Actually both should be permissible, as both can be true in the statistical and cultural sense.

We're not unique snowflakes as we like to believe. Our culture shapes us in many ways, nicely in some ways, badly in others, and it does the same to others sharing it.

There's a reason why stereotypes exist, beside bigotry and arbitrariness.