I'd say it is more likely to be a language design choice that decisions be explicit, not implicit. For instance, what if I wish to group the data in a different order than that in which I output the columns? Or if I want to group by columns that aren't included in the columns selected? Or if I want to output grouped columns only and not use aggregate functions? Only by explicitly stating my preferences in the group by clause are my intentions clear.
You also have to remember that SQL is a very old language (1970). Look at how Linq flipped everything around in order to make Intellisense work - it looks obvious to us now, but SQL predates IDEs and so couldn't have taken into account such issues.
