Validating check constraints without a table scan?

Is there a solid reason why adding a check constraint does not use existing
indexes for validation.

We are currently looking at partitioning a multi TB table leaving all
existing data in place and simply attaching it as a partition to a new
table. To prevent locking, we are trying to add an INVALID check constraint
first and then validate it.

I can trivially prove the invalid constraint is valid with a simple SELECT
which will use an existing index and return instantaneously. But AFAIK
Theres no way to mark a constraint as valid without scanning all the rows.

This operation is really problematic on a production database with heavy IO
load.

Is there a solid ready why validating check constraints cannot use existing
indexes? If I can prove the constraint is valid so trivially with a SELECT,
then why can Postgres not do the same (or similar)?