Being forcibly invited to leave home would be a motivation! However, we’re not doing that, just incrementally raising the cost of living here until it approaches the cost of living independently.
Speaking from experience being forcibly invited to leave home won’t motivate a male of the species to become a responsible adult. It will motivate him to find other sources of support, which might even include getting a job. But supporting yourself is not the same as being a responsible adult. My son is a prime example.