I know some of these people. We've had deep conversations about what is going on in our thought processes. Their description significantly differs from mine.
These people are common enough that you likely know some. It's just not a topic that frequently comes up.
It is also a spectrum, not a binary thing (though full anendophasia does exist, it is just on the extreme end). I think your own experiences should allow you to doubt your claim. For example, I know when I get really into a fiction book I'm reading that I transition from a point where I'm reading the words in my head to seeing the scenes more like a movie, or more accurately like a dream. I talk to myself in my head a lot, but I can also think without words. I do this a lot when I'm thinking about more physical things like when I'm machining something, building things, or even loading dishwasher. So it is hard for me to believe that while I primarily use an internal monologue that there aren't people that primarily use a different strategy.
On top of that, well, I'm pretty certain my cat doesn't meow in her head. I'm not certain she has a language at all. So why would it be surprising that this condition exists? You'd have to make the assumption that there was a switch in human evolution. Where it happened all at once or all others went extinct. I find that less likely than the idea that we just don't talk enough about how we think to our friends.
Certainly there are times where you think without a voice in your head. If not, well you're on the extreme other end. After all, we aren't clones. People are different, even if there's a lot of similarities.