I suspect that there is no mathematics exit exam simply because the government would prefer it if we didn't have proof at just how innumerate that our secondary school diploma holders are.

As far a I can tell the 'powers that be' prefer it when the population is innumerate. This way they can take them to the cleaners financially with high-interest rates, loans that they can't afford, government debt, green energy scams, lower electricity rates through long-term borrowing, etc. (Look at the what happened in the US financial meltdown in 2008 or the Ontario electricity system.) I've worked with lots of poorly educated immigrants and Canadians who were barely literate, but when it came to calculating their over-time pay or how to budget a household they could run mathematical rings around our modern secondary school graduates.

One of them most important lessons in arithmetic should be understanding the exponential function and compounding interest.  Low interest rates won't last forever.