I could be wrong, but I thought it was because you get a visa to stay for 3 months, which can be extended to 6 months if you can prove you're holidaying. I always thought it was because it's much harder to facilitate a new passport from outside your country of residence, so they work on the basis of it being valid for as long as the maximum amount of time you could legally stay in the country.....but I can't quote sources - just the reason I thought they did it!
All ESTA is, is an electronic version of the I-94W - that green form you used to fill-out on the plane, stating you weren't a Nazi or were on your way over here to overthrow the government. It is also your declaration that you abide by the rules and limitations of the US Visa Waiver Program. In doing so, you surrender your right to challenge the decisions of US Immigration, Customs and Border Control.
This is
not a visa and it doesn't actually entitle you to
anything. The CBP agent can grant you
up to 90 days admission - that said, he also has absolute veto - he could grant you 10 days, or he could decide he doesn't like the cut of your jib and refuse you entry entirely - and under the VWP you have zero recourse - visa waiver (
waiving the rights offered by a visa).
Nor can you adjust status, whilst using the VWP - i.e. decide you'd like to stay longer - or apply for a visitors visa (B2) that allows certain individuals up to 6 months per stay. However, if you qualify for VWP it is very unlikely that you would be granted a B2 visa.
I'm reasonably well versed in the subject of US immigration, so let me know if anything needs clarification