The Constitution requires the Supreme Court chief to preside in presidential impeachments. When Leahy, who at age 80 is the longest-serving member of the Senate, disclosed last month he would fill that role in Trump's trial, he noted the president pro tempore had historically presided over Senate impeachment trials of non-presidents. Trump left the White House last month after his Nov. 3 election defeat by now-President Joe Biden.
https://www.reuters.com/article/us-usa-trump-impeachment-leahy-idUSKBN2A91E9
Maybe OP believes that Trump is still President...
Well, he was elected
In 2016
And 2020