Real countries simply tell you how much you get back or owe at the end of the year. They have waay more data on what you've done than you do. No reason they can't do it other than the fact that it's profitable for their friends if they don't
The US could give you a potential number, but it's not accurate for about 10% of people who actually itemize in their tax returns. Yes companies lobby to prevent that, but the concept that it's hard to come up with that number is FUD. It would take the vast majority of people 15 minutes or less to prepare their taxes, if they could read and follow directions.
it would probably take most people 15 minutes to do their taxes if they were tax advisors. unfortunately for most people, the directions and wording of the forms is very confusing.