Sure, if you define property rights such that the US Government technically "owns" all the land in the USA then we are all just tenant farmers and taxation is not technically theft.

The issue is that we AnCaps dispute the claim that the US Government should own everything. Its claim was obtained illegitimately and without the consent of those it was taken from, starting with the Native Americans and followed by the private landholders who were homesteading out west and then suddenly found themselves under the jurisdiction of a "state" they never consented to.

The government is basically just a bunch of people getting together, drawing up borders, and claiming everything within it "theirs", even though people are already living there and getting on. How on earth is that legitimate?

If the government was founded properly e.g. with a unanimously supported Constitution then it would be legitimate. But it was not. The original US constitution was written by delegates so far removed from the people they were allegedly "representing" that there was virtually no consent. Furthermore, even if you accept the premise that the US Constitution had the consent of the governed, the US government doesn't even follow the original US Constitution! The 16th amendment, for example, was legally ratified, but it was forced through anyway by Wilson's administration. The government does numerous other things that are completely unconstitutional and against the spirit and letter of the contract known as the US Constitution that was allegedly consented to by everyone involved. Therefore no, it does not have the legitimate right to tax our wealth.

Furthermore, we can always overthrow the government and eliminate it and live in a post-state anarchy.