> Why can't it check if it's correct every time it's loaded?

This is considerable extra work and it's pointless. When you add together some numbers do you find yourself having to painstakingly re-assess whether the Peano Axioms (which make arithmetic work) do what you expected for the numbers you're adding? But what if today, this time, 2 + 2 is 7 or 5 instead of 4?