Why is sump pit built to bring exterior water into basement only do to be pumped away? The basement is dry except where the pit creates a penetration. Why invite exterior water into the basement?
The sump pump could empty a pit that collected water which found its way into the basement by misfortune.
Why not keep the basement shell waterproof like a boat and only pump if there is a leak?
Getting the basement to be a waterproof shell like a boat may be possible, but maintaining that waterproofness is very likely not possible over time. Ground shifts, concrete and mortar dry out, foundations crack, and water can come in. Other ground shifting can occur elsewhere and make a dry piece of land wetter from underground. Hurricanes and heavy thunderstorms happen, and seem to be happening more. The chances that a dry basement may get wet from water outside aren't down to 0.
Additionally, if a place exists for water to get close to the basement and get pumped away before the water gets into the basement, then that's good planning. Make an easy place for the enemy to approach and it'll be easier to stop the enemy.
It's a matter of risk assessment. Strange things happen when one is not looking. If one plans for as many strange things as possible, then one won't be as taken aback when strange comes calling.
It's quite possible to have an external sump, if that's put in at build time it doesn't even cost a lot more, since the basement area has already had a large hole dug to put the basement in. Most people don't...
Better yet is a drain sloping downhill to daylight that does not need a pump, so it does not depend on power to the pump - but terrain must allow that, and the ditch for the pipe needs to be dug at a small extra expense.
Many basements are indeed built with the hopeful, but incorrect (virtually always) belief that they will keep water out.
Then they don't, and water needs to be removed from below the basement so it does not get in. It costs easily 10X as much to install an external drainage system at that point, since everything has to be dug up again, so chopping a hole in the floor for a sump pit is a lot more affordable, and that's what's normally done.
Some builders build one in to the basement in the first place, presumably because their customers expect to see them, and explaining an external sump might confuse some, and anyway it would cost a tiny bit more at build time that would not be recouped at sale time, which is math that builders don't like.
You invite the water that is under and around your basement to collect in one place where you can easily send it somewhere else.
Waterproofing is preferable but not all basements can be waterproofed. Some examples: Ones with floors built without waterproofing, ones with old walls made of permeable stone, ones with below-grade entry doors for cars.
Sometimes a sump pit is a lot cheaper, and just as or MORE effective than waterproofing an existing basement with water problems.
"The basement is dry except where the sump pit creates a penetration" is incorrect. Without the sump pit the basement would be wet. The sump pit is part or a drainage system, usually perimeter drains that direct water away from the basement floor and into the pit. And yes, sure, without the rest of the sytem (drains) and without the pump, the pit all by itself would be an accelerator for water entry.
If you have high ground water at times, then you want to pump down that ground water to below your floor's elevation. The sump pit exposes the ground water so that the sump pump can lower it to an elevation below your slab. Without it, you would at least have a humid basement. Without it, you might even need a pump sitting at your basement's low point to pump away periodic puddles.
Done properly, a sump pit can be as well sealed as the rest of the foundation.
Note that even if your basement is watertight, and stays watertight (cracks can develop, and go unnoticed until there's enough water pressure for them to leak), there's always the risk of a burst pipe causing flooding from inside the house. A pump guards against that, and a sump pit let's the pump keep the water level from rising above the floor, as well as permitting the pump to run more efficiently.
In many cases, a sump pit serves as a collection point for drainage pipes, sometimes called "drainage tile" but generally made of plastic, into which water will seep if the ground around the basement is wet. Although it might be possible to make a basement waterproof, it's generally better to keep the ground around the basement dry than to rely upon the concrete to resist water pressure without any leakage. While it might be possible to have many pumps scattered throughout the drainage tile, it's generally easier to have a pit that's lower than any of the drainage tile, thus allowing water to flow out of the tile using gravity alone, and then use a single pump to lift all of the water that has entered any drainage tile anywhere.
Incidentally, such systems are also important for things like window wells. Water that enters a window well will generally have an easy path into the drainage system, allowing it to be pumped out via the sump. Even if a basement was completely waterproof, a pump would be needed if there wasn't anyplace below the bottom of the window well where water could leave the property by gravity alone.
