I think it's for political reasons, a fair number are indicted for relatively minor offences, that are trumped up to large offences so they will plead out. But if it's your first rodeo your going to assume the worst, the prospect of spending years in prison gives them the necessary rightous indignation to carry out there own death. In the USA system our judicial system has been eroded, and currently they are convicting people on only circumstantial evidence, which means there only burden of proof is to make it look like you committed a crime without evidence or proof. The jury's instructions are circumstantial evidence is just as good as actual evidence. Decades ago you had to be guilty beyond a shadow of a doubt, then some doubt was ok, now it doesn't matter if you are guilty or not as long as we can make it look like you could have committed a crime. Why? Because incarcerated persons is big business now and they could care less about guilt or innocence as long as half the state budget goes twords it,many of the judges own stock portfolios of the private contractors which own and operate these for profit prisons, the only way to increase it's value is to increase inmate population and build more prisons. Judges are very political and it is an elected office not an oppointed one, the public believes being tough on crime equals the number of people being sent to prison.