Why Is There A Statute Of Limitation On Rape?

Many U.S. states have a statute of limitations on rape which create an expiration date for the crime. Why do statutes of limitations exist?

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Bill Cosby and the case for ending statutes of limitations on rape (vox.com)
"Bill Cosby has been accused of sexually assaulting more than 50 women over a period of five decades, but so far only one case has resulted in criminal charges."

No More Statutes of Limitations for Rape (nytimes.com)
"THIS week, Bill Cosby was charged with three counts of aggravated indecent assault. The felony charges stemmed from an incident more than a decade ago, in which Andrea Constand says Mr. Cosby drugged and assaulted her in his home. "

Statutes of Limitations for Sexual Assault (victimsofcrime.org)
"This chart summarizes the applicable state statutes of limitations for the prosecution of various sexual assault crimes."

Why Bill Cosby isn't going to jail, despite his shocking testimony (vox.com)
"This week, the Associated Press obtained court documents that showed Bill Cosby admitted under oath in 2005 that he had procured Quaaludes, a sedative, to give to women he was planning to have sex with."