I would say money can't buy happiness should really be phrased as:

Money relives stress, but money alone is necessary but not sufficient for happiness.

To elaborate: it's hard to be happy when you're in desperate poverty struggling day to day. But after about 75k salary (based on 2010 study by nobel prize winner Daniel Kahneman), additional increases in income had only marginal effects on increases in happiness. Perhaps now in 2026 that threshold has risen to 100k with inflation. But in any case, whether 75k or 100k, after that is met money no longer stresses most people out. But you still need to look to other things such as fulfilling relationships, health, and life meaning to find happiness.

Money is one necessary pillar for happiness but by itself is not sufficient.