Yeah but remember that English is pretty irregular, so even though appending "-s" turns many words into their plural form there are many counterexamples. Like:

- one fish, two fish

- one sheep, two sheep

- one child, two children

- one mouse, two mice

- one knife, two knives

In this case many of us grew up using "lego" as the plural, so "legos" sounds to us a bit like "sheeps", "mouses" or "childs". Not a huge deal - nobody's losing any sleep over this, but it's definitely worth explaining why it sounds weird to some of us :)