An elementary question about French

How do you get into the situation where it’s correct not to bother with the second half of every word and instead to tail off into a formless vowel? How do you simultaneously find it correct to keep writing all those terminal consonants, and sometimes whole syllables, that you’re not supposed to say? I’m guessing the laziness of aristocrats has something to do with it. One thing’s for sure, though: sei pa bel ei sei pa cle.