most ppl don’t have much money and live month to month, as per the halacha these ppl are not only wealthy, but are entitled to tzedaka (less than 6 month cash reserve, makes a person entitled to tzedaka)
ad bli dai, dai means enough, translation is they’ll have wealth until they say enough, that’s literal translation.
all the gemarahs, seforim, rabbi’s, tanach, say the reward is wealth, not that you’ll be able to make a living
So we established the reward and promise is wealth, and that doesn’t mean being poor and happy!
The question of what is wealth? 100 million, 500 million? great question, I’m not sure. i guess it would depend on what makes that person “happy” (that’s what pirkei avos means happy with what you have) but in either case it’s not living month to month, or even 10k in the bank. maybe you can argue if someone owes zero on his house, car, and has money put away for all his kids weddings, etc. that’s wealth, fine. how many ppl have that kind of money?
not many, so we’re still back to the question of why not?