Why pagination with groupBy is not efficient in Laravel?

In Laravel pagination documentation here http://ift.tt/213C9hb, it is given that

Note: Currently, pagination operations that use a groupBy statement cannot be executed efficiently by Laravel. If you need to use a groupBy with a paginated result set, it is recommended that you query the database and create a paginator manually.

Just wondering why it is so ?

I am using group by for some of my reporting queries

via Chebli Mohamed