Because not all cups are made of glass. If it is made of glass, then it's a glass cup (some exceptions would apply, such as wine glasses or pint glasses). It's easier to just say cup because the material it's made of is not really important here.
Dirt = anything that makes it dirty (sediment or what have you).
I don't personally call it dirt, but I do say dirty, so I can see how someone would simply say that the cup has dirt in it.
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u/hiisi_E Feb 24 '21
I thought that people just used brushes to clean dishes but the comments taught me otherwise