The higher the value used the less impact is has current wise on the circuit. Remember there are 4 POTs in there that all use the same power rail. So 4 x 10k in parallel is really a single 2.5k load on there, but you toss in 4 more 10k and now it's a 1.25k load and now it is using twice as much current. Using a higher value, like 1M, it will still change the voltages of it, which is what the MCU is measuring to 'see' where the stick is, but current wise you've changed it from a 2.5k load to a 2.475k load, not a big enough difference to really matter.
Placing it in series is far more work as you would have to place it on the correct side, or both sides and use twice as many parts, and it would really stuff up the divider. Unless you used some trimpots with more turns that you'd want to turn so you could dial in that very minute value, whereas tossing them on there in parallel is quick and easy, and unless the stick is really way off it's not noticeable, and if it is way off then trying another stick or POT should be done first before trying to dial it back in with a parallel one.