 Of course, the fantastic book, Because Internet, by linguist Gretchen McCulloch, talks about this. And she talks about how it's not just the vowel, but sometimes we add extra consonants, even when we don't pronounce those consonants or even when it's impossible to lengthen those consonants. Like we might add more Gs to OMG, but you don't really say OMG to lengthen that. It's strictly a visual trick.