No, I think it's just because of an overall lack of centralization and the tendency for US federal agencies to continue to grow in power instead of coordinating

No, I think it's just because of an overall lack of centralization and the tendency for US federal agencies to continue to grow in power instead of coordinating with other agencies, resulting in a massive amount of duplication of functions and capabilities. In a well-designed government, the IRS would not have any armed agents. If they have an operation where they need armed support, they'd simply call up some other agency (perhaps the FBI, or even local police or SWAT teams) that specializes more in armed agents and operations, and partner with them to conduct the operation. IRS agents don't need to shoot at people; they can let some other highly-trained people do that, and then come in after the bullets have stopped flying and seize documents and analyze them.