Schools stopped doing that because students largely refuse to prepare. Testing throughout the year is like a CI pipeline and is shown to work better for the median student.
I think if they offered a proctored do-over a week later, the bad results on the first test might prompt them to make an attempt at studying for the next week, and the prospect of having to sit through two tests and getting shamed for having to do-over might prompt people to actually study for the first test.
Students are neither generally stupid nor constitutionally lazy. I sense that when expectations are clear they'll often surprise you with diligence. We should trust them to do the right thing. If they do, it's an A; and if not, it's less than that.