And yet everyone accepts that Julius Caesar was assassinated by a conspiracy of republicans.

How about Cicero versus the Catiline Conspiracy?

How about Prescott Bush and the Wall Street Conspiracy?

And what about Lincoln? With Appomattox not a week behind them and the Confederacy a stone's throw across the Potomac, where was his guard? Booth had his own motives, but Thaddeus Stevens and William Seward benefited.

We are fans of the show "NUMB3RS" and have several seasons on disk. In two of the episodes, FBI agent partners Colby Granger and David Sinclair banter about the Kennedy Assassination.
Granger: "... FBI, CIA, ... it would take hundreds of people to cover it up. You could not keep it secret."
Sinclair: "... Which is why everyone knows about it."

Was Galt's Strike, not a conspiracy?

Not every secret plan to rule the world really exists outside of the minds of those who think they discovered it... but some do...