Do you have kids?
Your brother in law did not merely save and not over spend, he built a business and sold it for a large payout.
Now our spending is much higher but the growth of our investments over the last 10 years has been incredible and outpaced that by a large margin.
For a while I thought some startup stock I had was going to fund a lavish retirement but then AI largely destroyed the value of those shares so we're just planning on the boring index funds.
I said nothing of investments and growth.