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One easy explanation is production traditions, the molds used for making beaker are older than dishwasher.
No one thought about this Problem, even today a good amount of people still do t have a dishwasher, so not everyone get to knowtice this issue at all.
If you look at the McCafe cups in europe, you will see that McD had them made with an interrupted rim on the bottom so they still sit flush on a table but the water can easily rinse of.
The manufacturer normally just don’t care.
Not an expert in any of those topics regarding cups, but Id speculate that for the case of pottery, its a product of the manufacturing process. In the production of pottery, a piece of clay is moulded wet, and then dried to set its shape. Due to the existence of gravity, the pottery will have to stand on something and dry that way. Then after drying, they fire the clay to make it water resistant. They will eventually have to scrape or get the dried clay off the thing it was sitting on while drying. If the bottom is flat, this would be problematic but if the surface in contact is a thin ring as opposed to a filled-in circle its easier to be released from the racks.
Plus, if its the same amount of material used, a dome is structurally stronger than a flat slab of material or feature of a design.
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