For me it goes like this:

Ok if we are already extending personhood to corporations, who with their sheer power transcend individuals, why not also extend that fiction to other entities that would actually need active protection?

Wouldn't corporations do just fine and we would live in a better world if we stripped any form of personhood from corporations? The biggest collision area stemming from corporate personhood is its collision with other, actual persons. The only reason corporate personhood is a thing is because it allows corporate lawyers to pick from a bigger pool of personal rights in a perversion of the spirit of these original rights. Thus watering down the existing right.