Earlier quoted context omitted.
Strange because as an American I've almost always heard to them referred as Legos. As a kid, my friends and I said, "let's play with Legos". My parents would say " get these legos off the floor". Seems odd to me to think of these phrases without the "s" on lego
I have a pet theory - in north america you have a popular snack called an eggo (pluralised to eggos) thats basically a waffle. I think americans started calling lego legos because they were already calling eggos eggos. In the uk and europe generally we don't have eggos, so we don't have legos.
More likely, the Americans lengthen the e in the first syllable into almost a diphthong, like /eɪ/, and shorten the "o" in the second syllable into a schwa kind of sound that can comfortably be followed by /z/. Meanwhile, in British and many European accents, the first syllable is short and strongly stressed /ɛ/, and the o is /oʊ/, and you can't add an s without contorting your mouth in some horrible way. Which is why British people are so disgusted by "Legos". Try saying it with an American accent.