It's not... the staff is underpaid for what they must do.You're right about US Corporations. I gave evidence in SFO's tax courts in 2008 on behalf of a US Corp against an IRS claim of $1.7bn.
The IRS settled for less than $200m. The IRS didn't seem very competent to me.
And it often gets told to "back off" by a purchased senator or congress-kritter...