For me, it's a matter of decency. If you're sending me an email and you're expecting that I'll read it, you're asking me to invest my time (presumably for your benefit). But, you're unwilling to make the same investment with your time by using a tool to simulate a human connection. Having excessively long, unedited prose adds insult to injury since you've doubled down on the decision to spend someone else's time as long as it saves yours.