If I understand correctly, in the United States, charges are currently being brought against President Trump on the suspicion of enlisting the assistance of foreign governments with re-election, in a procedure called impeachment.
There are no charges as the term is commonly understood, i.e. criminal charges. It is an Impeachment, not a criminal trial. Actually, it is not even that; currently, it is an investigation into whether or not to begin Impeachment procedures.
The President is not charged and not tried. The House of Representatives is investigating whether to start the process of Impeachment, which is a political process to remove the President from the political Office of the President because he is deemed politically unfit.
Why does this trial involve politicians?
It is not a trial. It is a political process.
Firstly, that would seem to violate separation of powers.
Charges are brought by the Executive Branch. The President is the (Head of the) Executive Branch. He would effectively have to charge himself.
As mentioned in the comments, this process is part of the Checks and Balances, and thus it is the exact opposite of Separation of Powers by design.
Secondly, those deciding on guilt are not neutral.
They are not deciding on guilt. They are deciding on political fitness to hold the Office of President.
In a regular trial, a suspect is considered guilty if the jury decides so unanimously. Following a US impeachment, the suspect is considered guilty if 2/3rds of the members of the Senate consider so. It seems reasonable to suspect that Senators may be biased based on their political alignment.
The President is neither a suspect nor will he be found guilty. He may or may not be found politically unfit to hold the Office of President. He will not be fined, he will not be put in prison, he will simply stop being President.
In Israel, Prime Minister Nethanyahu is indicted on the suspicion of corruption, but this trial is run by independent courts, not by fellow politicians. I am aware Israel has a parliamentary system and the USA a presidential system, but I don't see why that makes any difference.
The difference is that Israeli Prime Minister Netanyahu is indicted and tried, whereas President Trump is not.
Again, currently, the House is still in the process of investigating whether or not to even begin the process of Impeachment. But even if they were already in the Impeachment process, it would still not be a trial, and President Trump would still not be indicted.
Why are politicians involved in deciding on the guilt of other politicians?
They aren't. They are involved in deciding political fitness for Office of the President. Or, rather, they are currently involved in investigating whether or not to start that process.
Note that there is a veeeeery remotely related process in Germany: Members of Parliament enjoy Parliamentary Immunity from Prosecution. This Immunity can only be revoked by Parliament. So, in some very remote sense, also in Germany, politicians are involved in "deciding the guilt of other politicians". Of course, again, they don't actually decide the guilt, they only decide whether or not to make it possible for the Executive and Judicial Branch to determine the guilt of a member of the Legislative Branch.