Because how can wrong ever be "necessary"?
It's the whole myth of "necessary evil" again. If something is necessary, it can't be evil, and if it is evil, it can't be necessary.
"Taxation", being theft, is evil, so there is no way it can be necessary. "Taxing" someone is wrong- even if you wouldn't have the money to do what you want to do otherwise. It doesn't matter how you try to justify it, or what you try to establish as the apocalyptic consequences of letting people use their own money as they see fit. If your plan is so necessary, convince people that it is and let them fund it voluntarily. If they choose not to, it wasn't necessary at all.
And, there are plenty of other examples statists will try their hardest to convince you are "necessary", even as they admit the act would be evil if you or I did it without the sanction of "government".
It's dishonest in the extreme.
Now, if you admit your act is evil, but you feel it is still "necessary", go ahead and do it- taking full responsibility for your actions and accepting whatever consequences come from it. Including being shot in defense. Just don't tell me I have to go along and approve because you can't think of any alternative.