Might as as well argue against any form of VAT. I mean my income was already taxed, why am I paying taxes again?
Like USA where such thing doesn't exist?
You're right, it's not a VAT, they call it "sales tax" and leave it to their individual states to determine which goods and services are taxable and which aren't. Shockingly, most states do have a wide assortment of taxes that we generally subsume under VAT in Europe.
But sure, whatever. Let's keep splitting hairs.
Sharpie can't argue his way out of a paper bag, but the fundamental point is a valid one. VAT/ GST/ sales tax is a tax on income that has already been taxed once. I'm not a fan of estate tax or 'stamp duty' or other taxes against spending that has already been taxed once; but for example a reasonable Capital Gains Tax is a good idea since it taxes added value. The really fundamental problem is that the rich, especially the really rich, almost always have access to loopholes anyway so stuff targeting them seldom works as they can use Trusts, Companies, Partnerships, tax havens, or combine them all into a Double Irish variant; and the average Joe can't.
Things like VAT are also a deeply regressive tax if applied to staple items like food rather than just luxuries since if you do that it disproportionately targets poor people- who have to spend money on food, accommodation, clothes- rather than those with discretionary spending who can choose when and where to spend most of their income. Even if you exclude food and have luxury tiers/ brackets they're almost always inconsistent and still gameable by the rich; here you just bung as much discretionary expenditure into a 'business' (which desperately needs a new 8700k dual 1080 Ti system, so I can work at home!) and then claim the GST back, plus you can write off depreciation for your shiny new 100k Tesla against any income and any net loss against your personal income. It's a lot more difficult to game straight income tax or a CGT.