It's because the International Health Regulations confer the power to declare a PHEIC on the WHO's Director General alone, rather than the WHO as a whole (or any Emergency Committee established under it).

I wish there was more accountability for International Institution.

Saying "WHO Director Declare X" is obviously more accountable than "WHO said X must do Y" with a shadow comity deciding for 7 billion people and claiming it's not their fault when they are wrong...