River Cruise from Bruges to Amsterdam

Do they exist?

I don't think there are any that short, especially on US-marketed cruise lines. Maybe check European river cruise lines? I did find 5 day cruises on CroisiEurope Amsterdam to Antwerp (their website mentions Brussels and Bruges, but I didn't see any stops or excursions to those cities when you look into those cruise details.)
You might look into barge cruises? If you Google "barge cruise Amsterdam to Bruges", several boat/bike cruises come up, most 5-8 days. Maybe you could rent a barge on your own, but my very cursory internet search suggest rentals are usually by the week?
Or you CAN do day canal cruises in both cities (separately). Many options on viator, trip advisor, etc. Good Luck with your planning!

I Googled "Amsterdam to Bruges river cruise" and this came up first. There are others you can find the same way to compare.

Note that it's for 10 days, starting and ending in Amsterdam, it doesn't go all the way to Bruges, and if you look at the map you'll see that it's not exactly on a river.

Tulips & Windmills - 2024 Itinerary:

https://www.vikingrivercruises.com/cruise-destinations/europe/tulips-windmills/2024-amsterdam-amsterdam/index.html#noscroll

We did canal trips in both Amsterdam and Bruges. Each was only for a few hours. It was fun seeing things from the water. My favorite? The Texas flag flying from a high window in Bruges.

You are not going to find such a cruise. Bruges has two sea going canals (not navigable rivers), and they are not linked in a way which is navigable by large vessels-
Bruges to the north sea port of Zeebrugge (you can walk that in half a day), and Bruges to Ghent. From Ghent the navigable waterway is the canal to Terneuzen. That town/city is towards the mouth of the River Scheldt- turn right for Antwerp, and left for the North Sea.

So it is not an attractive route for the river cruise operators as it would be a lengthy dead end diversion from Antwerp with nowhere (out of Antwerp) to stop before Ghent, and then Ghent to Bruges is too short for an overnight passage.

You can certainly rent a barge from Bruges to Ghent- and in theory onwards to Terneuzen, More likely you would have to go there and back to Terneuzen, and return the barge in Ghent or Bruges.
You are not going to be allowed onto the Scheldt on a rental barge to Antwerp- as it is a very busy commercial waterway with ocean going ships as well as commercial river vessels, alsobpilotage is compulsory. You will only cover the Scheldt, as a tourist, on an ocean going ship.

Think like a cruise line, dock yourself in Rotterdam and say it is both Amsterdam and Bruges, plus Antwerp, Delft, etc. ;)

But as others are saying, there is no single interior passage from one to the other, and 2-3 days is woefully short if there were. I saw where Celebrity Cruises does like a 4 day cruise out of Southampton that lists Bruges and Amsterdam, but obviously does not dock in Bruges, and if regulations go through, nor Amsterdam, and would not be a river cruise.

I would also suggest adding some time, then piece together water and bike tours in the different areas. Certainly Bruges and Amsterdam have Canal tours, Rotterdam has water tours of the harbors, Antwerp as well. I believe you can also find some longer ranging day trips by boat on canals between Amsterdam and Rotterdam, even a ferry between the two I believe.

Bike tours along the canals, would also work, either rent a bike or find an organized tour.

Piecing something together yourself may be the best option.

Thanks everyone!

No company can make money with a 3-night sleep-aboard. That's not the River Cruise "product." Unfortunately, newsboards often use the word "cruise" to describe a day-boat ride.