I have tried checking online but no one seems to discuss this. It seems that the grain of sea salt is larger, but I don't understand since the salt is dissolved in the sea water so how does it become coarse.
The method of production of sea salt usually involves bulk evaporation of seawater. This allows the salt to crystallise and due to it's bigger size it is sold without much processing. On the other hand, the table salt has much smaller size and is processed a lot more than we could assume before it is sold.
This could be one reason why sea salt has coarse texture.
See also: Maxine Builder and Maxine Builder. What's the Difference Between Kosher Salt and Sea Salt? (via the Internet Archive)
I was interested to answer this question because I have always thought that both the salts are just the same. The tiny size of salt grains just seems less coarse but it essentially is having the same texture is what I would believe.
But then I looked at sugar as well. The tiny grains of sugar or a little bigger sugar diamonds seem smooth compared to the rough textured bigger blocks of sugar. And so I was convinced that it was due to it being processed. But then think of powdered sugar? That appears to be smooth.
Because it is not ground as fine , unless a customer wants finely ground sea salt. It is passed over screens to separate it into sizes after grinding, just like table salt. Fun fact ; table salt that is too fine for traditional marketing ,is sold as "popcorn" salt.
Sea salt is usually sold as coarse crystals because, as others have noted, it is separated by crystal size before packaging. Sea salt, along with Kosher salt, is desirable by chefs and food enthusiasts because it allows one to see how much salt is actually put a piece of food, let's say a steak, without dissolving as smaller crystals would do. And it gives a desirable crunchy mouth feel which many people like. With some foods it is desirable to add salinity to saliva as the food is about to be chewed. This causes more saliva secretion and enhances the food's flavor as it's broken down by chewing. It's important to know if you're using Kosher or sea salt or regular table salt when measuring salt for a recipe because salt with large crystals will take up the same volume but weigh less than salt with small crystals, such as common table salt. So a teaspoon of one contains less salt than a teaspoon of another.