I guess this comment I'm replying to was downvoted for being too vague and blunt, but it's a perfectly valid point.

Let's say my monthly expenses (in areas that might theoretically have coupons - e.g. house rent isn't going to come with a groupon offer) are $2k, then saving 25% of that is only ever going to net me $500/month, and there's a hard cap based on how much I'm already spending.

Whereas with investments, there's no cap on it, except the amount I have to invest, which could be much, much more than $2k.

Obviously this skips over the question of success in those investments, but that isn't the question being discussed.