How big is your empire? If you're a major force on the map you will find it hard to get alliances, and so the same applies to marrying off your princesses. You'll never live to see the other factions getting a claim on your lands -- by definition it will only happen after your (male) royal line dies out and your game is over.
On occasion you might have a claim on the lands of a faction whose princess one of your princes is married to though. It's never happened for me, but there are people in the Org who've won in that particular lottery!
The only other thing I can suggest is that maybe your princesses are all old trouts with body odour, too much facial hair, and green teeth
edit: and welcome to the org, btw!
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