Because the legislative branch is closer to the people.
The authors of the Constitution were mindful of taxation without representation - a situation in which a government imposes taxes on a particular group of its citizens, despite the citizens not consenting or having an actual representative deliver their views when the taxation decision was made.
The power to collect taxes cannot be delegated by the legislative branch to the executive branch because that would separate the taxation and representation functions.
The legislative branch is Congress (also considered as the voice of the people) is created by Article I, Section 8 of the Constitution.
Its main job is to write laws. It doesn't interpret the laws (that job belongs to the Judicial Branch), and it doesn't administer the laws (that job belongs to the Executive Branch).
Supplement your own homeschool American history or government curriculum with Cahsa.info's U. S. Constitution study guide.