The reason is Delphi-compatibility. Delphi has them masked the same and this way code ported from Delphi will meet the same behaviour.
And again: integer division is not handled by the FPU, thus it is not handled by the exception masks either, because these are only for FPU based operations.
My point in showing default FPU masks was only to indicate that some math masks (regardless of FPU or integer) are on by default. Thus, someone originally must have felt that such masks weren't "exceptional".
There is no "regardless of FPU or integer", the exception masks in unit
Math are
only for FPU exceptions. There is
no mask for integer exceptions.
Also, as I said, the set of masked exceptions is due to Delphi compatibility.
[…] The reason is Delphi-compatibility. […]
Ahh! “Our FPC developers on earth, hallowed be thy Delphi-compatibility, […]” 
You can make fun of it all you want, fact is that the majority of the developers that come to FPC are those that had experience with Delphi (or sometimes TP) and thus this compatibility
is important. Otherwise we'd have many more users complaining about (in this case) not receiving certain exceptions or receiving exceptions they didn't expect.
Although, to be fair FPC has {$overFlowChecks} [for integer arithmetics] by default off.
For “Delphi compatibility” I guess. 
Also due to performance. There would be more people complaining about the code being slower by default than there are people that are missing the checks.