I don't think I'm suddenly going to accept that the data presented to my function is valid and fit for purposes just because it goes through someone's JSON schema library. That's just passing the responsibility on to a third party, which feels lazy and dangerous to me.

Do you also distrust that third parties can implement HTTP, generate SQL, enforce the constraints in your DDL, do arithmetic operations, etc. correctly ?

Of course there is still a lot of semantic validation you have to do manually (e.g. for an order: does the referenced product exist, is the seller allowed to sell it, at this time, for this price, for this currency, taxes, do the line prices add up to the order price, contingents, etc., etc.).

Expressing these constraints in your interface definition is just another declarative validation layer like your implementation language's type system, database schema/constraints, generic validation libraries[0].

Also, as the server author you are usually also the author of the interface definition containing all these constraints.

[0] E.g. https://beanvalidation.org . The OpenAPI to Java generator will express constraints using BV annotations. BV also integrates with the Hibernate ORM to validate data before you send it to the DB, or you can manually annotate your DTOs/domain model and programmatically trigger the validation at any time, so you only have to know one set of annotations and rely on a single library for validation in any layer of your application.