That old "no taxation without representation" kind of thing. Many and Several States have no desire to dabble with Federal Hocus Pocus ala Taxes... Nor any Desire or Need to Compel those Residing Within A State for Failing to Declare a Taxable Purchase Made in Another State or Location.
Long and short, it is not the Seller at fault here, it is the Buyer, who is (in most US locations, and probably a few over seas, too) duty bound to declare "I bought this, it cost this much, and I own this much tax. Here it is."
The Buyers aren't doing their job, and politicians don't really want to hammer those who elect them, so are going after the companies/biz, which are NOT REQUIRED to collect taxes outside their jurisdiction...
Freetards Untie! That's what's brought us to where we are now...and it is not going to change anytime soon.