Why do roasted tomatoes taste sweeter than raw tomatoes?

Answer: Water cooks off and sugars brown.

Acids evaporate, leaving only sugar behind. — Some acids can escape during roasting, and that can make the tomato taste less sour. But acid loss does not leave pure sugar behind; the sugars were already in the tomato and become concentrated as water leaves. This option mistakes a drop in sourness for an increase in sugar.

Starch turns into sugar in the heat. — Tomatoes contain little starch, and roasting does not turn much starch into sugar. Heat can brown the sugars that are already there, but it does not create new sweetness from starch. This option gets the importance of heat right but uses the wrong starting material.

Water cooks off and sugars brown. ✓ — Roasting drives water out of the tomato, so the sugars already present become more concentrated. The heat also browns sugars and breaks some of them into smaller, sweeter-tasting compounds. Some volatile acids may escape, which lowers sourness, but the extra sweetness itself comes from concentration and browning.