I understand all this But, why do manufacturers like ford sell bigger engined cars then? Whats the point to them?

Ie why sell a focus with the choice of 1.6, 1.8 and 2.0 litre engines?

Why not all be 1.6 with the choice of 1.6 std, 1.6 bigger air flow, 1.6 even bigger air flow...???

Gen question...