Market value of carbon offsets drops 61%, report finds | Negative scientific and press reports on the efficacy of carbon credit projects has led to a ‘direct pullback in buyer investment’

The big companies buying have been doing in full knowledge that the offsets are fraudulent — it's an instrument for greenwashing. The public exposure of this reduced the value of offsets for that purpose, which is why they're not worth as much, even though they're likely to still be largely fraudulent.