Just 50 states would be easy. You're forgetting county and municipal/locality taxes. Or the varying due dates for each of those jurisdictions. Or whether or not your widget business creates a nexus in those states and requires you to register as a foreign entity. Or whether or not your specific widgets trigger a higher or lower taxation rate. Or that whether or not shipping should be taxed also varies state to state. Or...

It's a whole lot of work to be legally compliant.