Because it is simply too complicated?
Tax credits wanted to know DH'a earnings for 2 years ago. Asked the tax office who kindly provide a list of his earnings. Fill in form. Make small mistake. Letter arrives from tax office confirming that tax credits cross checked with tax office, revealing small mistake in my math.
So, if you can cross check. Why can't you talk to each other so I don't have to fill in the form? Remove the middle person & all the crap. No more miscalculations. No more rubbish overpayments.
Bah