Because the bubbles are lighter than the liquid of the drink. They form at the bottom or sides of the glass because they use the roughness of the surface to collect together and form, then once they’re big enough they just let go and float up. They also get bigger the higher they go, because the pressure from the water is less when they’re less liquid above them.
Yes the simple answer is the bubble is less dense (its lighter), it’s exactly the same with helium balloons floating up or hot air balloons, The helium or hot air are less dense than normal air so they rise.
It’s also to do with how pressure works – because pressure gets higher as you get lower in the drink, there’s more pressure on the bottom of the bubble pushing up than the top of the bubble pushing down. So the bubble rises!