Code requirement for burying electrical PVC that runs along foundation

The trench actually needs to be deep enough that the TOP of the conduit is at least 18" deep.

Frankly, you'd be better off paying the slight upcharge for schedule 80 the whole way regardless, because it IS a slight upcharge and you would need schedule 80 for the parts coming out of the ground anyway, and it's a lot harder to break.

The difference is exactly what @isherwood's comment calls out - once it's buried, it's hidden and non-obvious, and "the rules" are that conduit hidden underground needs to be protected. Some landscaper planting rosebushes does not expect to find a power conduit 6" underground, and you won't always be around to tell them it's there, so there are rules.

There are ways to go shallower, but they cost serious money (Using rigid metallic conduit, pouring concrete cover that extends beyond the conduit) and are usually not practical for most people as a result. The conduit on the exposed face of the building can be seen and avoided, and is required to be at least schedule 80 (below 8 feet) as insurance against accidental impacts.

If the issue is that you don't want to see it running on the outside of the house, the simplest (and shortest) solution is usually to run it inside the house if you are heading to "the opposite side of the house" - drywall repair is trivial compared to ditch-digging, and you can use EMT or (I'd personally not) NM cable for the "inside the house" part of the run (still need wet-rated wire outside.)