Not a doctor, but the temporal lobe is the part of the brain in charge of memories, language, and a whole bunch of other stuff. The key thing about this is that memories and languages are in there, and association of memories to words.
When you look at something, you think of it as a verbal word because you've spent so long associating that thing with that word (that's how you learned how to speak, just by associating things to words, and figuring out how sentences get strung together just by hearing enough sentences). The human brain is super good at looking at something (for example a water bottle) and pretty much instantaneously labeling it as a word, and it's always doing it.
And you do this all the time. Even when reading, you'll visually look at a word, and the voice in your head will read it back to you as if you are saying it in your head. Try it. Check out this word. Potato. Read that word. Potato. Can you almost hear it in your head? Po ta to. Crazy stuff. It's called subvocalization.
Your brain is just wired to try to take all these memories of things, and turn them into words, and convey them. Like me, right now, I kind of have a grasp of how this works, so my brain is just rifling through all sorts of of memories and thoughts of all these phenomena that I can try to use to help explain this to you, and I have this thing in my head that's trying to string all sorts of words together in such a way that makes sense if I typed it out and you read it. And it's kind of an auditory voice (not really a real voice, but a concept of what the words would sound like if I said them instead of typed them, because my brain is just taking all this crap and turning it into a different format. Instead of pictures and memories, it's all just getting converted into words and what words would sound like, and then they get strung together into sentences.)
And most everyone does this, even when they aren't trying to convey. If you are taking a dump and reading a shampoo bottle, your brain is still doing it. Taking image, turning into a different format. It just happens all the time, ever since day 1. Eyes see things, ears hear things, brain tries to put all that together and label it. Humans usually speak to express things, so it becomes the preferred medium, and after some decades of doing it, it just becomes so ingrained that it feels "normal".
Some people don't do this, deaf people might think not in audible words in their head, but rather images or even muscle memory of signing words, but they do it too.
People that have suffered traumatic brain injury or otherwise caused their temporal lobe to get messed up may lose this ability as well, it's called Aphasia, and it's when someone has an impaired ability to speak, or even associate their thoughts in their head to words they know.
A neurologist could probably explain this a lot better than me, but humans are just social creatures and our brains are wired to take things in one format (sensory input) and convert them into another format (language) so that we can relay those things to other humans, and we're so good at it that we do it all the time.