Because you can buy your way into a light bike/ components- loosing weight off the body takes effort and commitment?
Basically that.
It is important, as are aerodynamic considerations. It all makes a considerable difference.
BUT, not the differences the marketting hype would have you believe. But, that hype is there to persuade you to part with your hard earned.
Spend £xxx on this, and you will float up the hills is their message because they are in the market of selling you stuff. They don't make money from you getting up at 5am each morning and heading off for a couple of hours of interval training.
Sure, you'll float a little better than you did when your setup was 200g more - but, unless you put in the effort and work hard, it will still be hard work.
Don't get me wrong, everyone should strive to ride the most aero and lightest setup suited to their purposes, but, as ever, don't believe the hype. A larger, unfit rider is still going to find hills hard work whether they are on a 10kg bike or a 7kg one.