Because snowflake doesn't build foreign key indexes. Imagine clickstream data where every insert is being checked against an index of customers. This isn't a typical usecase for big data warehouses.
That said, Snowflake does support constraints if you use hybrid tables (a preview feature announced at their last conference).
Tools and scripts can work off of it, design decisions are documented, suggestions can be made, inferences can be made (some dangerous, some not).
Why tag S3 objects if it doesn't enforce a schema? Maybe a bad analogue but I'm going quick right now :).