EU members have passed extensive tax harmonization over the years. Rates are set by country and the details of what's taxable where and for how much has been made consistent throughout the union. No such thing has happened or is practical in the US. Every state has its own rules for what's taxed at what rate. For example, in most states you don't have to pay sales tax on food items bought at the grocery store, but in others you still have to pay sales tax on those items fully or in some states at a lower rate than you would for non-food items. In New York, you don't have to pay sales tax on clothing. North Carolina has an annual sales tax holiday where certain items like books are sales tax free for 3 days a year.
What really makes it complex though is that the rates or rules are not consistent across an entire state because local governments can also have their own taxes. Where I live in Seattle, we pay the base 6.5% Washington state sales tax plus 3% in local taxes. Some of those local taxes go to the city where the item is purchased, some to the county where the item was purchased and yet some goes to the regional transit authority spanning multiple counties. On top of that, if you're buying food and beverages in a restaurant, there's an additional 0.5% which goes to pay off debt for stadiums.
There are tens of thousands of different individual jurisdictions just like this across the country. Harmonization would mean elimination of dedicated funding sources for local governments which is just very unlikely to happen any time soon.