Because you can't always understand their attempts at words, and when that happens you both become frustrated.
As adults, when we're not understood we have a plethora of things we can do. If you were in a foreign country for example you might mime, point, show on your phone or even draw your message. Kids can do some of those things but they need your prompts to do so.
The important thing is that we keep their confidence high and show them that we really want to get to the bottom of what they're trying to communicate. There's nothing more frustrating for a kid than not being able to communicate.
What do you do when you can't understand your child? Any I've missed?
Watch the full reel here