Ask HN: Does separation of DAL and Business Logic create simpler applications?

Having worked in software in a few different domains, I've bumped in to this question quite a few times with different companies doing this different ways.

The traditional n-tier / mvc / DAL mindset is that data validation needs to happen in a set of native objects, and once that logic is evaluated data can be updated in the persistent data store.

Why is this preferred over maintaining data integrity at the datastore level, and having all the API endpoints getting to behave as pure RPCs with data retrieval and business logic mixed together?