time at work when the kids are young, you get to spend significantly more time with them throughout their lives.
By working hard when you're young you get experience, knowledge, capital, titles, connections etc that you can then leverage for decades. This leverage allows you to spend less time at work while you're still making more money. 1 + 1 = 3
The time spent is an investment in your future. It's a sacrifice (mostly of personal time and hobbies - it need not be sacrificing time with family) that pays off later.