I did some work for the IRS several years ago to explore the same idea. There are many benefits, including simplicity and accuracy of filing (closing the $500B tax gap), as well as better fraud protection (another $25B). Protectionism of the tax return industry is common, but not the primary reason. Instead, the biggest pushback is from taxpayer advocacy groups. The issue is this: for many Americans, particularly low…
If your sole income is 1 main full time job, it's really not hard to estimate your taxes to the point of having a refund under $500. That way you don't government use you as a bank for 0% interest and have more money throughout the year.
To preempt some of the responses - no, let's not denigrate poor people for their spending habits and pretend like somehow getting the refund is better than having that money during the year.