When entering combat the daily buffs disappear visually. Basically the Icon denoting the buff disappears but the effects are still applied. This only applies to the daily buffs that come from resting, all other buffs/debuffs visuals still appear.
I'm going to make a totally random guess here: Because it makes it easier to see the most immediately relevant effects without half the field or more being taken up with passive buffs that aren't going to expire in combat, or effects that are completely irrelevant to combat.