When you say “they,” who are you referring to?
As for why the Bill was written....
So, what is behind the fact that the Bill of Rights did not really amend, and was not intended to amend, the Constitution? Why write it, then? It helps to realize that James Madison is known as “The Father of the Constitution.” He is also known as “The Father of the Bill of Rights.” There’s a clue in that.
Madison was, understandably, a partisan of the constitution that he had written. The Anti-Federalists were not. They were vehemently opposed to the new federal government created by that document and were pushing for a bill of rights to radically alter it, or even for a second convention to create a new government entirely from scratch. Madison did not want to change the constitution that he had written, so he wrote a bill of rights that indeed did not change in any meaningful way “his” constitution. Simple.
It was written for no reason other than to fool people. The Bill changed virtually nothing, and that was precisely the intent of the man who wrote the Bill. As one author noted, the amendments were “carefully crafted to change not one word of the original Constitution.” The Bill’s author had no desire to change the Constitution, and the Bill was created to fool those who desired a bill of rights into thinking that he had produced one.
Madison’s amendments were not designed to fool Anti-Federalist leadership, men like Patrick Henry, who, in fact, rejected the Bill of Rights, precisely because it changed nothing. The amendments were, however, meant to fool the masses, the people. And to the extent that the people really did want the Bill (there is debate as to how motivated the people really were for amendments versus how much the push had to do with powerful, elite leadership by the likes of men like Henry), Madison succeeded.