Because the assumption is that "entry level" means someone who's still living with Mom & Dad; or various other situations. (Leaving jail, recent immigrant, mother returning to the workforce.)

IE, someone in an "entry level" position is someone who has partial support in their household.

Of course, there's plenty of good discussion to have about the ethics of this kind of employment. Needless to say, I prefer to spend my dollars are stores where "entry level" employees are teenagers, retirees who just want to get out of the house, and people who need an extra opportunity to make an honest life.

So, assumptions that are no longer true, and haven't been for a while?

Because economics

No. If you can't pay your employees a living wage, that's your failure, and you shouldn't be in business.

Some jobs generate less value for businesses than the arbitrary amount you deem to be a "livable wage." That's economics.