As others have said, the reason for this is that the US Constitution grants a jury trial as a right in criminal proceedings.
The reason for that right to be granted is to be a check against (that is, a limitation of) the power of the government, generally, and judges, specifically.
To answer your specific question of why "the judge system is not reformed and stays like this and is more vulnerable to take a wrong decision because they do not know the law, thus the verdict depends on people’s morals?"
Firstly, the jury does know the law (or at least, the specific portions of the law that are relevant to the case), by the time they render their judgement. An important part of the judge's responsibility in a criminal case is to craft "jury instructions" that are given to the jury before they are sequestered for deliberations. These instructions should have an explanation of what questions they need to collectively answer, and what evidence they can and cannot consider.
Secondly, there are two central dogmatic difference between the points of view that your question seems to promote or be based on, and the one generally held by the US Founding Fathers, who wrote the US Constitution. Your question seems to assume that:
- judges are sacrosanct, or at least trustworthy; and
- rule by law is the foremost concern
In contrast, the general view of the US Founding Fathers was that:
- judges have power, and thus need to be checked (in the US system, a judge's power is checked by the prosecutor, the jury, the chief executive, and by courts of appeal); and
- protection of the citizenry from the power of the government is the first concern.
One thing to note, is that this second point means that the judge in a US trial can actually override a criminal jury, but only in the defendant's favor.