parenthesis and relative clause

I have a question about why a parenthesis is necessary in a sentence instead of a relative clause:
Why it should be:

Such an impact, many scientists believe, would have blocked the planet with debris.

Instead of:

Such an impact many scientists believe would have blocked the planet with debris.

I think I can understand the second one as "Many scientists believe such an impact would have blocked the planet with debris.

byw, it is a question from ACT English.