I think they just look at tax related crimes rather than any crimes.
Sometimes accurate tax records can provide evidence of crimes which are not tax related.
There's no reason to believe that the IRS would know about the implications of the numbers outside of the scope of taxes.
The IRS wouldn't know if the accurately reported income was the result of an illegal activity.
e.g.
If I make $50,000 selling gold I mined, and I claim that $50,000 income for tax purposes, me and the IRS are cool.
Looking at my tax records, they won't have the wherewithal to know if I mined that gold from someone else's mine.
But an LEO who knows that my mine is tapped out, would be able to look at the income from mined gold and see that something was ****y.
So, the fact that the IRS has not tried to have Trump arrested (or w/e it is they would do) is not a guarantee that no crimes were committed.
At best, it's an indication that IRS has not seen cause to take any action.
Though, I think the first action the IRS would take if they saw something ****y in someone's taxes would be to begin an audit.
IRS apparently has begun an audit of Trump.
It seems the results of the audit inform the IRS's next step. 'Cause just because something looks ****y, doesn't mean it necessarily is ****y. If the audit shows everything is decent, then the IRS closes the case. If the audit shows that the stuff that looked ****y actually IS ****y, then the IRS will probably keep the case open instead.