The boys' blessing uses the exact wording Yaakov said we should or will use for our blessings:
"And he [Yaakov] blessed them that day, saying: 'By thee shall Israel bless, saying: God make thee as Ephraim and as Manasseh.'" (Breishit 48:20)
There is no such specified blessing mentioning women, so when people decided to bless girls as well, they made something up, and the 4 mothers were the natural choice to use.
There is actually one relevant example in Tanach of a blessing mentioning women:
"And all the people that were in the gate, and the elders, said: 'We are witnesses. The LORD make the woman that is come into thy house like Rachel and like Leah, which two did build the house of Israel; and do thou worthily in Ephrath, and be famous in Beth-lehem; and let thy house be like the house of Perez, whom Tamar bore unto Judah, of the seed which the LORD shall give thee of this young woman.' " (Ruth 4:11-12)
But maybe that focuses too much on fertility, and we care more about other things, like righteousness. Also, the story of Tamar is not "nice", even if Tamar came out of it honorably.