Short answer
Because they represent little or nothing more than transaction and marketing costs.
Long explanation
In the United States, voluntary RECs are predominantly supplied from jurisdictions where they are not eligible to be sold to electric utilities for RPS compliance. Here, RECs sales are considered a small source of income (e.g., subsidy) to electricity generators, yet have been shown to not provide a sufficient incentive to alter renewable energy generation investment decisions (i.e., lack of additionality). The difference between the retail price of voluntary RECs versus lower wholesale prices reflects added transaction and marketing costs.[1] The simple answer is that supply of these certificates vastly exceeds demand (see If more companies purchase RECs and GOs, then won’t this increased demand eventually cause more renewable energy investment and generation?).
In the United States, the wind Production Tax Credit (PTC) and solar Investment Tax Credit (ITC) have been shown to meaningfully influence RE investment.[2] Also, RPS compliance REC prices have also been shown, in jurisdictions with ambitious quotas, to meaningfully influence investment and generation. [3] Recently in the United States, new renewable energy generating investments are accounting for most new generating capacity and becoming least-cost new capacity, in part due to government subsidies and mandates.[4] This market trend will likely keep voluntary REC and GO prices low, absent of a nation-wide (federal) RPS or clean energy standard.
[1] U.S. EPA. The Benefits and Costs of Green Power. Guide to Purchasing Green Power. https://www.epa.gov/sites/default/files/2018-08/documents/guide-purchasing-green-power-3.pdf
[2] National Renewable Energy Laboratory (2014). Implications of a PTC Extension on U.S. Wind Deployment. https://www.nrel.gov/docs/fy14osti/61663.pdf
[3] National Renewable Energy Laboratory. Renewable Portfolio Standards: Understanding Costs and Benefits. https://www.nrel.gov/analysis/rps.html
[4] U.S. EIA. 2021. Renewables account for most new U.S. electricity generating capacity in 2021. https://www.eia.gov/todayinenergy/detail.php?id=46416