It's a function of anatomy. The sublingual veins drain into the jugular and then vena cava. For drugs this avoids the first-pass effect, which is exploited when giving nitroglycerin for angina pectoris. Other ways to achieve this are suppositories (rectal veins) and intravenous (i.v.) or intramuscular (i.m.) injection.
The liver will eventually remove ketamine from the bloodstream regardless of which route is used.
That's my understanding at least.