If we were to keep it in the mouth any longer, it would become very unpleasant. Maybe we feel very hot and go to take a cold shower. Nothing but absolute misery would result. We can enjoy a cold shower for ten or twenty minutes at the most. Anything that is prolonged will create dukkha.
All contacts pass quickly, because that is their nature.
The same goes for sight, our eyes are continually blinking. We can't even keep sight constant for the length of time we're looking at anything. We may be looking at a beautiful painting for a little while and really like it. Nobody could do that. We can't look at the same thing a long time, without feeling bored, losing all awareness, or even falling asleep.