Because whether or not something is strictly illegal is not the only standard we hold in this country. If congresspeople were not only complicit but actively helping the capitol rioters intentionally or otherwise, we need to know. We need to know how this happened start to finish and how to keep it from happening again. We need to know if the threat still remains, and we need to know who is fueling it. The FBI can find fa tual info on this to give to the committee, and the committee is tasked with pulling that and all other info together into a bigger picture.