Y'know, for consistency! (And so the dubbed episodes release sooner.)

Not all of them speak English, they likely have contracts which keep them busy with other projects, there's a process to translate and have them make sense, then an editing process to make sure the translation lines up with the timing of their mouths so that they're not moving while staying silent, then lining up the new voice lines to the new edited animation from a different studio.

In other words, it takes time and they can't "just do it."

Really, from what I see, what they say in Japanese already syncs with what they say in English, at the very least they shouldn't need to change THAT.