Michelle was 17 when she married Jim Bob. So there is another example.Josh's wife was 20 when she got married, so marrying young is what they do. However, from what I've seen on the show, none of the girls, with exception of a couple, seem all that into kids and marriage. Who could blame them? And beisdes, If the older girls got married, that would mean Michelle would actually have to take care of her own children.
But it isn't like the girls can even go sign up for Christian Mingle. Potential suitors must go through Jim Bob to even approach the girl about a courtship. That man only sees dollar signs. While Josh and Anna make money for the family by being on the show, any daughter who gets married would essentially move away to be with her husband and his family. It isn't like there is an enclave of Gothard following people there in Northwest Arkansas. A child missing for an episode or two wouldn't be noticed, but one moving away would be noticed by viewers. Yes, there could be visits and vacations, but it would be a different aspect that I'm not sure Jim Bob could see the monetary value in at the moment.
Many who are in the movement do value their privacy, modesty and humility. There is a hypocrisy with the Duggars' television cameras, news show appearances and publicity seeking that does not sit well with a young man who might want to marry one of the daughters. While the show portrays them as the royalty among overly fertile people, I'd think that quite a few "godly" young men would be turned off by the cameras, attention, and potential for being used.