Fertilizer, herbicide, and pesticide runoff, lawnmower (gas, maintenance, wear and tear, etc.), water wasted on keeping the lawn green, to name a few.
That said, if your “lawn” is like @Creekland’s natural pasture only occasionally requiring a “haircut” with a ride on mower and does not use watering, chemicals, etc. - that is perfectly fine. Just like our “swamp.”