pardon limitation–“except in caes of impeachment”

Trump’s pardon power is vast, “except in cases of impeachment”

To stop Trump from going on a pardon spree designed to pardon potential witnesses against him in an impeachment inquiry or proceeding, why can’t the House of Representatives simply initiate an impeachment inquiry?

In a similar vein, if the President issues pardons to witnesses who may be called against him in a criminal investigation, this would appear to constitute obstruction of justice.

Clearly Trump could be prosecuted after he leaves office for committing the crime of obstruction of justice. Whether his pardons would be effective vis-à-vis those he has pardoned is a separate question. There are powerful legal arguments supporting the conclusion that they should and can not be effective.