The Constitution says so. Fact is, however, it just doesn't work that way. The American justice system hasn't been fair in years - if it ever really was at all. Presiding judges arbitrarily make decisions based on their own opinion, not the facts in a case. Court-appointed legal representation regularly lacks the resources and influence available to lawyers in private practices and giant firms.
Justice is increasingly dispensed haphazardly, with little regard to existing evidence.
Social values and economic privilege have pushed justice into a dark corner. They don't belong in the courts at all. Not when the future of a defendant rests on subjective codes of moral and public acceptability. Not when a particular type of offense automatically dispels juror impartiality. Not when people are convicted for what they might be instead of what they might have done.