Why does white chocolate usually taste more like sweet cream than like bitter dark chocolate, even though both trace back to cacao beans?

Answer: It leaves out cocoa mass

Sugar locks bitterness away — Sugar can mask bitterness, but it does not explain the missing flavor source. Dark and milk chocolates include cocoa liquor or nonfat cocoa solids; white chocolate is built from cocoa butter instead. A very sweet dark chocolate can still taste chocolatey because the brown solids remain. White chocolate's surprise is that it keeps cacao's melting fat while leaving behind much of cacao's bitter, aromatic chemistry.

It leaves out cocoa mass ✓ — Right. Cocoa mass, also called cocoa liquor, carries much of the brown color, bitterness, phenolics, and methylxanthines that make dark chocolate taste unmistakably chocolatey. White chocolate keeps cocoa butter, which is mostly a fat phase, then adds sugar and milk solids. Food-chemistry measurements show dark chocolate has far more phenolics and flavonoids than white chocolate. That is why white chocolate can be legitimately cacao-derived while tasting like the cacao flavor has been turned down.

Milk cancels cacao aroma — Milk changes flavor, but it is not a chemical eraser. Milk chocolate still tastes like chocolate because it contains cocoa solids along with milk. White chocolate tastes milky mainly because the cocoa solids are absent, not because dairy somehow neutralizes them. The useful comparison is milk chocolate: it proves milk can soften cacao flavor without deleting it.