Actually it would.
Very slightly. Very very slightly. As a matter of fact, the weight of the house’s materials will far outweigh any bouyancy effect, so it won’t really move.
But in essence, it’s correct that a sump is not truly necessary if the house’s basement floor is above the groundwater level. However, if you have a rain event that exceeds the ground and sewer system’s ability to remove, the water will look for otehr places to go, which is likely to include that basement.
Here’s the thing: you’ve got a whole bunch of water systems in your house; as we all know, water flows down. So the waste systems all collect at the lowest point of the house. This includes any basment floor drains or toilets you may have, which are at the basement floor level. Thus, they have to flow downhill a bit yet to drain properly.
If the water table is higher than the basement floor, water will come into the basmement through several means: the walls, any cracks, the joint between wall and floor, even around the floor drain. This can be dangerous because the water will then go into the waste system and can mingle with your waste stream, bringing the Bad Stuff (known in the business as black water) back into your basement. This happened on a huge scale in New Orleans.
A sump pump takes this water and pumps it out into the sewer system or septic. A sump pit or crock allows enough water to accumulate to activate the pump (they get damaged if they run with no water) without flooding the entire basement, but in sever storms, the water may come in faster than the pump can push it out.
Sometimes a pump is also necessary if the sewer system is at a higher level than the lowest point of the waste system. This is called an ejector pump, and it has to handle the waste stream as well as the water.
You can, actually, build a house waterproof. But it won’t stay that way due to things like workmanship, movement, wear, and the other penetrations in the waterproofing you have to make. It is not a system that you want to rely on. If the water table is high, a sump pump is simple and cheap.