Why? If side-projects are just that, hobby tasks, then it's a toss up of spending $X disposable income on things I/my family enjoy, vs $X - $20. $20 is an additional day out with the kids, or half a pair of shoes for one of them... or an extra $20 overpayment on the mortgage (worth so much more in the long-term).
Bottom line: it has nothing to do with income, but what people value.
Bottom line: it has nothing to do with income, but what people value.