Sweetness is not the “polar opposite” of bitterness. If it were, bittersweet tastes would be impossible to achieve - the bitter chemicals would neutralize the sweet chemicals, until all you could taste would be whichever remained unreacted.
Sweetness is an indicator of the presence of sugars (notwithstanding the few things like stevia, that trick us into tasting sweetness without the presence of sugar). The polar opposite of that is the absence of sugar - not bitterness, just the absence of sweetness.
Bitterness is an indicator of the presence of chemicals with other chemical characteristics, unrelated to being sugars. The polar opposite of of that is the absence of bitter substances - not sweetness, just the absence of bitterness.
If anything, you could say bitter is the polar opposite of sour (acids taste sour; alkaline things tend to taste bitter, and they really do neutralize one another).
