> Maybe they got spoiled by ORM mappers that anything beyond standard CRUD may feel cumbersome to write.

A decent ORM[0] will use the constraints expressed in the application to reflect those as constraints in the DB schema as well.

Why the duplication, you ask? For the same reason that many of the same constraints are imposed in the front-end code: Responsiveness.

Validation that happens in the front-end avoids a request/response to the server just to give the user feedback that their password isn't long enough.

Relationships and constraints expressed in the application can avoid querying the database just to get the same kind of feedback.

This duplication may seem useless, pointless, repetitive, and redundant, but not all accesses to the web server will be via a JS-enabled client, for example anyone accessing the application via an API.

[0] eg. SQLAlchemy: http://docs.sqlalchemy.org/en/latest/core/constraints.html