Whether or not I factor in tax is entirely dependent on the size of the purchase I'm making. For the vast majority of purchases I make on a daily basis, I don't think about it at all. 7% of $2.99 is negligible to me. However, if I'm making a large purchase, or if it's a purchase which I know is subject to additional taxes beyond the sales tax, then I might consider it. 7% of $2.99 may be negligible, but 7% of $29,999 is a significant amount all by its lonesome.

For most people, I'd imagine this is most common when it comes to purchasing vehicles, as those tend to carry large prices and special taxes which results in a significant increase in price. For example, I purchased a new vehicle a couple years ago for MSRP, but wound up paying several thousand dollars more than that due to various taxes and the registration fees. I didn't know exactly how much those surcharges would be (though I easily could have calculated them by visiting my state's Department of Motor Vehicles website and plugging a figure or two into their calculator) but I had a ball park idea which I could budget around. Also, I'm pretty sure the dealership I bought from provided an estimated total purchase price which included the fees for the locality it was located in. Unfortunately, most of that was irrelevant to me, as I had traveled from another state to purchase this vehicle, which illustrates the minor frustrations that an all-inclusive price tag would introduce in America.

Like, I don't think you're wrong for thinking it's odd, and yes, there are ways to fix it, but it's just such a non-issue (not to mention America's "touchy" relationship with taxation meaning these attempts to "fix" things would rapidly become politicized) that no one cares to do anything about it. As someone else said, we intuitively learn what the rough tax rate will be for our common purchases and just factor that in.