Why is compression rarely included as a builtin effect?
While you often see compression in the effects section of editing software, it is dynamics rather than actual effects.  In the traditional sense in the live world, compression is not considered an effect.  The software world simply does it that way because they are all software filters, but even then they are often in a dynamics category.

Compression, Expansion, Gating and Limiting are all dynamics.  I'm not sure if the effects hardware could do compression, but the controls for the parameters on the effects are certainly too limited.  You need attack, release, ratio, threshold at least and gain and soft/hard and auto attack/release options are common.  You can't adjust these with the very basic effects controls provided in the board.

There are boards that have built in compression, but they list it under dynamics normally.  There are also stand alone compression units you can get that you can use the inserts to feed the signal to.