The reason she is not for adding to the IRS is the Democrats have a history of building up the bureaucratic state, which then is used to target political opponents. Bureaucrats are not elected but they allow the Dems do things they would not vote for, publicly; regulatory state. If the IRS was used to target Trump, they could pretend they just woke up and this will be looked into. Luckily, the Supreme Court is starting to see this trend and is forcing Congress to directly vote for things, now assigned to unelected officials, so they can be held accountable at the ballot box.
What would be better than adding an army to the IRS, would be to simplify the tax code, so you get things done with less IRS personnel. This is how you reduce deficit spending. The complexity of the current tax code, which is needed for the leaders to get campaign donations and acquire golden parachutes, is making the job harder for the IRS. The Democrats do not wish to simplify this, since this will reduce their loopholes used for quid pro quo and direct skimming. How do you think leaders get rich on a civil servant salary?