Ketchup remains on the bottom of the container because it is purposely designed to be a 'non-Newtonian fluid'
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Ketchup, which is made by heating ripe tomatoes, filtering them, boiling them at a low temperature, and then adding sugar, salt, vinegar, etc., is a seasoning that goes well with hot dogs, sausages, omelets, french fries, etc. There is a notorious defect that 'remains on the bottom of the container'. Scientific American, a popular scientific journal, explains that such ketchup defects are caused by a physical phenomenon called 'non-Newtonian fluid.'
Ketchup Is Not Just a Condiment: It Is Also a Non-Newtonian Fluid --Scientific American
https://www.scientificamerican.com/article/ketchup-is-not-just-a-condiment-it-is-also-a-non-newtonian-fluid/
For example, margarine does not spill when it is in a container, even if it is turned upside down. When you look at the situation, you may think 'Isn't it hard?', But when you insert a butter knife, you notice that it is a soft substance that can be scooped out with almost no force.
This phenomenon is due to the fact that margarine has the property that its viscosity changes depending on the force applied. Margarine has a strong viscosity when the applied force is small, but it becomes low in viscosity when the applied force is large. In fluid dynamics , a fluid whose viscosity does not change depending on the applied force is called a 'Newtonian fluid', and a fluid whose viscosity changes depending on the applied force such as margarine is called a 'non-Newtonian fluid ' and is treated as a separate fluid.
The reason why ketchup remains at the bottom of the container is
Ketchup is purposely designed to be a non-Newtonian fluid because of its usage and texture. High viscosity in the unforced state enables the usage of 'melting and not falling when applied to foodstuffs', and low viscosity in the applied state means 'in the mouth when chewed'. It creates a texture that 'melts out with.' Ketchup is designed to be a non-Newtonian fluid, so when you put it on a hot dog, it melts into your hands and clothes and does not run off, but only when you put it in your mouth.
Apart from ketchup, shampoo is also a daily necessities designed to be a non-Newtonian fluid. When the shampoo is dropped on the palm, it retains its viscosity, and when it is rubbed on the hair, it loses its viscosity and blends into the entire hair because it uses the properties of non-Newtonian fluid. The same is true for toothpaste.
In modern ketchup, plastic containers are the mainstream, but in the past there were only glass containers, and a larger amount stuck to the bottom. Regarding modern plastic containers, Scientific American commented, 'Although it has become easier to handle, the sportiness of removing the sauce from the bottle and the pleasure of removing it cleanly have been lost.'
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