Warning: this post contains myriad assumptions based on impossible-to-know numbers
According to rough calculations taken from Star Wars: The Essential Atlas, there are about one million member worlds in the Galactic Republic, each with an average of two billion inhabitants, meaning that the total number of citizens in the Galactic Republic is about two quadrillion (2,000,000,000,000,000). Even assuming that only 70% of this two quadrillion is of tax-paying age (based on Earth age structure demographics), this is still an unfathomably large number of taxpayers. For comparison, in the U.S., there are approximately 225 million people over 18 and about 75,000 IRS workers, meaning that each IRS worker handles the taxes of roughly 3,000 citizens (obviously this is not completely accurate as not all those people pays taxes nor does every single IRS worker handle taxpayers). The Galactic Republic equivalent to these U.S. numbers would necessitate ~466 billion tax workers to account for the totality of the Republic citizenry. And while this number also seems ridiculously large, one could argue that it is all a matter of scale and that this process could instead be completely automated.
However, even if the Galactic Republic Revenue Service does employ that many workers, it still will have a major tax evasion problem. Don’t want to pay your taxes this Galactic Standard year? No problem, you have literally an infinite amount of space to go to. And unlike in the U.S., where international agreements allow the IRS to see your information if you flee to certain other countries, in the Star Wars universe, there are many non-Republic planets that would not know or care if they were sheltering tax evaders. Space is just too big, and the Galactic Republic bureaucracy, despite its potential size, simply cannot cover all of the ground that Earth-based tax agencies can. Sure, evaders can succeed in both scenarios, but it is much easier to avoid your Galactic W-4 than your U.S. one.
Earth, despite its vast size and various nooks and crannies, still is finite. And so for a tax evader looking for places to hide and a revenue service looking for places the evader might be, the total number of destinations on Earth is capped, whereas in space, that number is infinite. So the IRS should thank its lucky stars that it conducts business out of Washington D.C. rather than Coruscant.