It's not about efficiency, it's about quantity. Let's say I profit $20 for each customer my ad engages. If I reach 1000 people with amazing targeting and get a 10% response rate, I'm making on average $2/person. If I broaden the reach to 100,000 people and get only a 1% response rate, I'm making on average $1/person.
Clearly, bringing in $1/person is less efficient than bringing in $2/person, but at the end of the day you have $100,000 instead of $2,000 (ignoring the cost of the ad for simplicity).
With the broad campaign, you still get the people you would have targeted with the narrow campaign. So if you're going to do the broad campaign, you're already done.
Don't forget to factor in agency costs and in house costs. People are expensive and hurt margins. At that scale it adds up since I'm sure many of their agency partners view them as a big brand advertising cash pinata
Clearly, bringing in $1/person is less efficient than bringing in $2/person, but at the end of the day you have $100,000 instead of $2,000 (ignoring the cost of the ad for simplicity).
With the broad campaign, you still get the people you would have targeted with the narrow campaign. So if you're going to do the broad campaign, you're already done.