Might not need all the complexities that you learn for Year 12 maths in everyday flying but in my opinion the better pilots I've seen have good basic mental abilities.
The airlines need to see that you learn quickly and apply what you learn. So the boffins in HR have determined that passing Year 12 shows that the candidate might have what it takes. Need to remember that airlines are run by business type managers these days.
If you head overseas, you'll need at least a high school education if not a degree to get a work permit in some countries.