I would say it's because the same words are the basis for most of your thoughts.
It's not just that we think something and then find words to express it. The two processes are more linked; the vocabulary we have influence our ideas. Once you rely enough on language, it's a huge part of the way you organize and apprehend the world. This is why it's so important to develop our vocabulary. In a way, the more words you know, the more thoughts you can have.
