There are some interesting effects with how armour and penetration work. LAV is not tough enough to penetrate heavy static armour while medium pay allot of extra to kill the static unit so are not an efficient weapon to use from a cost perspective. Add in that static is much harder to hit and it will be very cost inefficient to use medium vehicles to destroy static fortifications. MAC weapons probably are them most efficient at destroying heavy static units but still not efficient enough.
I'd agree medium vehicles vs. static MAV isn't a great match-up for the vehicles.
What is a great matchup against static is LAV infantry.
Not it you have a CAP or HCAP in the Static instead it is not...
In terms of defending then Static will almost always be the best possible answer. Against infantry with LAV weapon then a static with no armour and an CAP is straight up better if both are fortified. You can always find a version of Static that is better than anything else depending on the weapon you put in to it.
If you then also mix troops then the Static usually shines even better as you can combine its good offensive and defensive ability for both cost and size well with especially infantry which enhance their killing power with the protection of the infantry against heavy weapons.
There is a few instances where infantry usually are straight up better utilising certain weapons and that is if they are specifically trained in Mountain, Jungle or Rift Valley training and using lighter weapons as the infantry will hit twice as often. On such planets statics are only better using heavier weapons in the defensive line which the infantry can't carry.
YMMV, when I look at those numbers I see the medium vehicle with 2xMAV being not that much more than double the cost of the static, and having nearly four times as much resistance against light weapons as one of the static units.
Double MAV medium vehicles are allot more expensive than Static units so you can get more than twice the number of Static units or you could imagine more both infantry and static units as a combination of units can often make allot of difference and enhance your strength considerably. This is why you generally have vehicle with a mix of weapons woth both anti infantry and anti vehicle.
Where vehicles really shine is their power to weight ratio which in terms of attacking power. On the defence then actual weight almost have no impact on anything other then mixing troops and here static are still quite space efficient for the cost of both infantry and the static units itself.
I would also add that you are not really going to see two fortified sides every fight, in fact I would not really allow that in my games and I would not abuse a passive NPR so I can fortify my troops and then fight, that is a bit abusive of the game mechanics (in my opinion). In multi-faction games I certainly don't allow that when factions are on the same body at the start of a war. If you play like that then statics are even more powerful as a defensive structure.