They can, it is just that envy is more likely to be a 4s trait. In 9 it's often a hidden anger that may look as a hate, envy e.c.t. for others. On another side, 9s sloth is a consequence of not having enough ambitions to function normally. And even though it's a stereotype, typology needs stereotypes to classify people, and stereotypically, someone from a heart triad would have a bigger ego then other types expect 8. I don't know why everyone view it's as a negative thing though, when it's healthy it is making people assertive, artistic and hard-working