While I agree that planning retirement based on income instead of expenses is Stupid, with a capital 's', let me offer a small defense of it, and hopefully everyone won't jump all over me, but recognize it may make a little sense for some people.
It doesn't for most on this forum because we LBOM. MOST people though live AT or even above their means!
If their lifestyle while working is paycheck to paycheck (as it is for the vast majority of Americans these days, from what I understand), their expenses level IS their income level.
So it could make sense to start there and say "Okay, I'll need less in retirement because no commute, etc etc, so 85% of my income" or "I want to do X, Y, and Z, so I need 135% of my income."
Now yes, those authors could still put expenses and have all those LAtYM people put their income, however most of those Americans don't know their expenses!
So while yes, I wish every article used expenses for planning, for most Americans (who don't know their expenses and live at their means) it is easier and quicker (and yes, lazy on the author's part) to say income. It may be the only thing that makes sense to all of those people.
Even though it makes no sense to us. It's easy to forget sometimes we're in the small minority, because everything you hear here is in such strong agreement.
For example, everyone bashing using income to plan retirement.
Still a bad idea, but hope I showed you it could make a small amount of sense for people not on the ER forum. Don't flame me too hard.