Max ABV in boiler

[Alcophile]

Hi All,
I've read on here that max ABV in the boiler eg after stripping runs, should be about 40% for safety reasons.
My question is why is this? The auto-ignition temperature for ethanol is Ethanol 365 °C (689 °F), you're unlikely to acheive this in the boiler. At the top of a reflux column of course the ABV is much higher. If coolant failure occurs then you get dangerous ethanol vapour released (especially with a column), not much related to abv on the boiler.
So why not have as high an abv as you want in the boiler as long as there is enough water so it doesn't expose the element?