These are later books and, unlike Pride and Prejudice or Sense and Sensibility, the young female characters, for the most part, do not NEED to get married. Elinor and Marianne (and Margaret when she comes of age) are impoverished gentlewomen who need a husband who can support them in the proper style; the Bennet girls are not poor now but would be if their father dies. Charlotte Lucas is facing a future more unpleasant than marrying Mr Collins.
But Emma is mistress of her father's house and is the highest ranking woman in her little town - marriage would bring her nothing material or social that she does not already have (and given who she eventually marries, her position changes very little indeed. It doesn't even bring anyone new into her life, really.) Harriet Smith, too, has been supported by her natural father and is in no danger at present of being poor. The only one for whom marriage would be a salvation was Jane Fairfax, who would other wise spend her days as a governess - except this was more her choice than anything else, as her foster parents seemed to love her being with them.
As for the young ladies of Mansfield Park - marriage for the Bertram girls was freedom. (Especially for Maria, who I am convinced worked her way through her husband's menservants after she got that pesky virginity out of the way - if she waited until marriage. Yeah, I'm not fond of her, since she sold herself to marry a man she didn't *like* and for no real good reason.) They had money enough, and there's no word of entail so they would never suffer if they didn't marry, and as the daughters of a baronet, they had social position as well. Fanny, too, was assured of a home forever, and was probably also provided for by her uncle.
On the other hand, we also have Mary Crawford, who is one of the paradoxes of this sort of society. She's immensely wealthy in her own right - 30K pounds, probably invested in the 5 percents. She should be completely independent, but as a young never-married woman, she can't be. She is essentially homeless, spending her time visiting people. Even during the time she lived with her sister, she's referred to as a guest, not an inmate. The same could be said for Miss Bingley, who when the novel begins is keeping house for her brother, and so has a home of sorts. This will change when Mr Bingley married Jane, rendering his sister, again, a wealthy homeless woman. The same could probably be said of Georgiana Darcy, but she seems a young sixteen, so this isn't really a factor at the moment.
However, except for the need for a permanent home, Miss Crawford can still afford to wait for an eldest son or, if her brother and Maria Rushworth had just behaved themselves with discretion, to marry Edmund , trusting on their combined incomes to live comfortably.
Meanwhile, are there any other people who try to rewrite MP such that the essentially boring and rather incestuous marriage between Fanny and Edmund doesn't happen? (If only Crawford and Maria had not eloped...)