[imtringued]

If everyone uses a different unit then it may not appear to be a lot at all. Nobody is measuring office space in football fields so how am I supposed to know how a football field sized manufacturing plant compares to an office building?

[cptskippy]

In general conversation, or general news articles people don't need exact units of measure because they're not acting upon that information. You use an approximation like football fields because it elicits a connotation and can help people easily grasp the scale.