Setting a Budget - Discussion

The Federal Governments fiscal year 2019 budget was $4.45 trillion (just in case you have trouble grasping the size of the budget it's 4.45 million million dollars. Basically what the Federal Government does when establishing its fiscal budget is to start with last years budget and adjust for inflation and population growth and any new programs that congress passes. What do you feel would be some of the drawbacks to this method?