Because the legislatures power to impeach and convict isn’t dependent on the judiciary.
Criminal and civil charges are a judicial branch thing. Impeachment is a legislative branch thing. The legislature does not answer to the judiciary, and the judiciary doesn’t have the power to tell the legislature how or when they execute their constitutional authority. Basically the only restriction is that the need some manner of “due process”, or to be basically fair.
There’s the office of the president and the individual who is the president. Both are often called “the president”.
In this case, it was ruled that the individual cannot be criminally charged for doing actions defined as a role of the office in the constitution: constitution says the president can veto bills, so a law saying it’s criminal to do so is unconstitutional.
There are other activities listed, the “official acts” bit, that are to be presumed to be immune unless you can prove otherwise, like the president communicating with the justice department.
The ruling didn’t change the ability of the office of the president to be sued or constrained, only delineated when you can legally go after the individual. “Delineated” because this has never been relevant before, so it didn’t matter that we hadn’t answered the question.
It’s a bad ruling not because it makes the president unremovable, but because those “other official acts” are given way too much slack.