This is because consumption today is always more valuable than consumption in the future (the discount rate).
This is because consumption today is always more valuable than consumption in the future (the discount rate).
If consumption today is more valuable than consumption in the future, why would you want to favor doing something else today, e.g. investment?
If the discount rate is 10% (not bank interest, just how much I personally value time) then unless you offer me more than $ 110, I'd rather spend the money now.
For people to invest, discounted_expected_return[1] - capital_gains should be higher than the money in their wallets.
You can play around in excel to understand this better, with a 5% return, a 20% tax on both income and capital gains and a 10% discount rate, $ 100 in income is either $ 80 today or $ 76 in a year.
[1] Discounted for Time, Expected for Risk.