Because often times you don't remember the domain exactly & then you've got the same problem. Anyone remember whitehouse.com or countless other examples of this? The fact that Google is probably 99%+ effective at getting you where you want to go heavily disincentivizes relying on your brain which may not have as high a success rate as Google. Plus all the browsers have gone in this direction since search terms are more usable than domain names.
Of course. This could have happened to any site, but in this case I just found it funny because 'JQuery -> jquery.com' is so simple and obvious. I'm sure the author knew the actual domain, but used google out of habit or preference.