Because it is a rule of legal procedure. For their consideration, the Grand Jury is only presented,with the basic PROBABLE CAUSE necessary to indict the defendant of the offense. Grand Juries never see all the subsequent evidentiary proof, which is eventually presented at a full-blown trial, therefore there is nothing to 'discover' from their hearings.
In many states they do, but in some they don't for felonies. These deadlines are known as statutes of limitations. Also what you're asking is if the prosecutors have a deadline to file charges. They don't indict, grand juries do. Prosecutors will either file charges by information, called a charging document, or seek an indictment by a grand jury.
Grand juries and Petit juries are the two kinds of juries. Grand juries review evidence of criminal action to determine if there is probable cause for bringing charges and if so, issue an indictment. Petit juries review evidence in both civil and criminal trials to determine the facts and render verdicts either for or against the parties in civil actions or guilty or not guilty in criminal actions.
The early grand jury system involved a group of citizens that investigated crimes, asked questions to witnesses and prosecutors, and more or less acted as a "portal of justice." Grand Juries refused to indict persons based on insufficient evidence, malice, or sometimes bad law. Colonists used the grand jury system to prohibit the prosecution of those who violated the Stamp Act and other controversial British laws. Today, in contrast, the Grand Jury system is effectively…