Because it has so many defects that “special interests” (people who have something special to gain that is not available to everyone else) are taking advantage of to interfere with, or even deny, the rights of other people, that a large scale amending is needed.
Which Amendments?
Amendments To The Current United States Constitution That Would Have The Largest Effect On Limiting Government And Increasing Individual Liberty
The Opponents Of An Article V Convention Try To Scare Us Like Little Children With The Bogeyman Of The Convention Going “Runaway”. Is This Even Possible?
A Runaway Constitutional Convention Is Impossible