Well, part of that is a choice to consume at a rate that requires such work. It's somewhat easy to have a nice financial position by delaying kids and not pouring money into cars and shopping ... but not many people choose that lifestyle.
The FIRE community is all about the alternatives.
And those are unpayable for many people without working overtime, even if they don't consume anything extra.
That's the only place the money can really come from to invest more than I already do.
Because the necessities have been going up at a far faster pace than the digital pseudo-luxuries.
Just as an example, my projected health care costs for 2017 went up by an amount that is larger than a nice phone + eating out every other week combined.
That's just the increase, with no appreciable benefit to me in the services provided.
I'm sure many people could provide similar examples with rent or other necessary services.
Everybody in the community has a different view of things, but for many "retire early" simply means "work on what you want to" rather than "bum around the beach all day for the rest of your life".
There are large subsections that focus on travel, giving up office jobs for being outdoors, and those that are truly much too frugal for my personal tastes.