SoCal,
Probably for the same reasons it's possible to obtain a driving licence without understanding the definition of the term 'coefficient of friction'!
Think about it, a few hours in the air (maybe, I don't know?), chat with the instructor, not too many 'veils a la Class D' (!) around... quite plausible for somewhere in the region of 80% of your flying theory to remain untested.
Then again, it all falls down to requirements, and I aint sure about other countries but I'm pretty sure there's no theoretical requirement to revalidating/renewing a flying licence (of all calibres) in the UK?