Why does this pocket still exist today?

Over time, pocket watches disappeared and were replaced by wristwatches, then by mobile phones. One might therefore think that this small pocket would have disappeared as well.
But it remained, simply because it became a traditional element of jeans. It's a historical detail that brands have retained to preserve the garment's authenticity.
Today, jeans are worn all over the world, by all generations, and this little pocket is part of their identity, much like metal rivets or thick stitching. The small pocket on jeans has thus become an iconic detail.