It is because sales tax is highly irregular, highly local, and changes often without any coordination across the multiple entities that can impose sales taxes. Manufacturers control published pricing for their products in advertising, physically on the container, etc. They are not subject to these local retail taxes, which can change the price >10% between locales.
This discrepancy is intrinsic to the fact that manufacturers set and advertise prices and then local governments effectively modify those prices unilaterally. At some point the customer will have to contend with the reality that the price at the manufacturer and the price at checkout don't match and control of that is spread across many organizations.
I'm not from the US, so i'm honestly curious, does sales tax really change that often?
The reason i'm asking this, is because here in The Netherlands there was some news a couple of years ago about changing tax on fruit and vegetables. Our tax authorities basically said: Nope can't do, we need a year to adjust our software for this. This should give you an indication of how often our tax rates change here.
It depends on the locale but usually not more frequently than once per quarter. Aside from this, one aspect that makes it complicated and intractable is that the rules for applying sales tax to various goods sometimes require very late binding -- the correct tax rate cannot be known until checkout because it is dependent on customer decisions at checkout. The common example of this is the definition of "prepared foods", because something you are going to eat immediately gets taxed differently than something you are going to take home. Tax authorities have standards of evidence at checkout based on customer behavior for food items that could reasonably fall into either category.
An argument could be made that sales tax should have rules that don't require this type of very late binding. The issue is that legal authority over this is completely decentralized as a Constitutional matter and there are thousands of overlapping tax authorities. There is no feasible way to compel them to all do something sensible, so the system has to operate under the assumption that some tax authorities will issue these types of rules as a matter of robustness.
>I'm not from the US, so i'm honestly curious, does sales tax really change that often?
It changes more often than you'd expect, but it's not daily. Perhaps yearly, but certainly across space too. Government in the US is very "layered." Our states are basically autonomous countries free to set their own taxes. Additionally, many states give sales taxation authority to municipal governments. There's little coordination across states or municipalities, so you might pay 7% sales tax on restaurants and 1% for groceries, then drive across the river to a different city/county and pay 6% and 2% respectively. There are also sales tax holidays where the tax is suspended for a given period within a state, but only for certain items.
Basically, it's very complicated due to the lack of coordination between levels of government in the US.
This discrepancy is intrinsic to the fact that manufacturers set and advertise prices and then local governments effectively modify those prices unilaterally. At some point the customer will have to contend with the reality that the price at the manufacturer and the price at checkout don't match and control of that is spread across many organizations.