The reason we can't speak is because we haven't learned and practiced enough.
To be able to speak without halting, your brain must have already encountered all the words and structures involved enough times to recognize them instantly and automatically. This is quite different than just knowing, which might involve a "process" of tracing back to something else for full recognition.
You can compare it with swimming. Until the part in the brain that controls your body is fully steeped in the mechanism of the required coordinated movements, you can't swim. You have to reach this critical point where things click and start to work. For language learning, it means knowing and practicing a very large number of words and phrases. If you want to speak with 300 different words, you'll need to already know at least 20 times that for example, because only a small fraction of what you know is actually usable when you speak.
In short, you have to read and listen until your brain accumulates enough English to be able to speak it fluently without thinking for seconds to look things up internally. It take lots and lots of hours to reach that point. So just keep learning and practicing until that happens.