An open braise evaporates and concentrates; a sealed one does not. That single difference is most of what separates a Dutch oven from a slow cooker.
A Dutch oven's lid isn't airtight the way a slow cooker's is, and even a well-fitted one lets steam slip out over three to four hours in the oven. Water leaves, salt and flavor stay behind, so the braising liquid gets thicker and more concentrated on its own, without anyone adding a thickener or reducing it separately afterward. That's the whole mechanism, and it's why gravy comes out of a Dutch oven closer to finished than it does out of a slow cooker.
What to do: leave the lid slightly ajar for the last thirty to sixty minutes if the sauce still looks thin, or pull the meat out and reduce the liquid harder on the stovetop once it's resting.