They are assuming that you've become accustomed to spending at that rate - commensurate with your salary - and that you'll continue spending at a similar rate in retirement.

They account for lower expenses for the work commute, work clothes, etc. I'm not sure if they account for having your house paid off or no longer paying into a retirement account.

But at the core - you're right. It's about expenses - they just use salary as a rule of thumb because too many people have no idea how to estimate their future expenses.