Why isn't the 2S38's APFSDS load limited similar to the OTOMATIC?

Both serve the same role in real life, why the disparity?

Different countries, different guns, different manufacturers, different ammunition…

An anti-aircraft gun would not carry 148 rounds of APFSDS no?

One of the noted things of the 2S38 is its limited ammo, which is why it has a dedicated resupply vehicle.

Why ingame can it take a full load as if it’s an IFV when its dedicated role IRL is SPAA?

No they don’t serve the same role. The 2S38 is still expected to defend itself against enemy armor as it is intended to provide a short range air defense but is still IFV. The Otomatico is an SPAA that is intended to primarily attack air targets and has the ability to load darts in the absolute worst possible case of an enemy breakthrough.

Meanwhile the VCC-80/60 got only 32 rounds and you talking about limited ammunition. xD
The simple answer is it’ a russian, underrated, premium tank that generates a lot of money for gajin. Because of this you can overlook that it should have the class “SPAA” and not light tank.
Maybe they will adjust its Br once the cow is milked enough.