That would be a perfectly reasonable convention, except it's already a convention that they do the same thing.
replyHaving them be different could cause someone to look at -h, and not even know about --help. Or if someone writes a script parsing the output of -h for some reason, someone else might come along and change it to --help expecting it to be the same thing.
The rust crate clap which is one of the more well known crates for command line parsing gets about 2M downloads a day and has this convention built in.
replyThis convention existed before clap came into being, but I don't recall when I first saw it. I have been using the command line for just shy of 40 years across various operating systems.