Because it's the right thing to do you know exactly what you pay in tax which means you are more likely to hold the government accountable for that money as well as being able to actually lobby for lower taxation.
Since sales tax is applied on both state and local government levels it also simplifies pricing both nation wide and even state wide.
Walmart for example sets a price for product A that price is the same across all states (for this example I don't know if Walmart has regional prices to adjust for cost of living but I don't think so).
45 states have sales taxes these come at different rates, about 40 states also allow for local sale taxes meaning that local county can set up it's own additional sales tax this means that the end price is different between states and even within states with local sales tax collection.
As a consumer you know exactly what you are paying if the product costs 100$ then you pay 100$ for that product and you pay additional tax for the sale when you know your state taxes you 8% and your local county taxes you additional 4% you can hold them responsible for what your taxes are being used for, you can lobby for lower sale taxes and you can always move to a jurisdiction with lower sales tax.
The way the sales tax is collected also allows for more convenient exemption from sales tax especially in non B2B purchases a sales tax exemption certificate can be filled at any purchase and they will not charge you sales tax.
You can literally go online print the exemption certificate for your state e.g. http://www.tax.ohio.gov/portals/0/forms/fill-in/sales_and_us... go to a shop pay the sticker price and fill it out and the store will not collect the sales tax on that sale.
Compared to how annoying it is to get VAT refunded and just how many caveats there are for getting your VAT back it's 100 times better.