Because it's too simple. Market distortions shift the price people pay away from what the true market price would be in a competitive market.
As an example, I live in one of the US's few competitive ISP markets. I pay $40/month for gigabit fiber, no limits. Comcast, which is often a monopoly or a duopoly, charges $90-140/month [1] for gigabit service, and they have assorted restrictions that I don't have to deal with, like data caps and limited upload speeds.
So what's the true market price for full gigabit? The person you're responding to, bassman9000 would apparently say it's $140, because tautologically that's what people pay. I'd say it's much lower.
Further evidence to support my position is that when you measure actual speeds delivered on gigabit, many of the ISPs are local providers, not the big nationals. [2] Customer satisfaction with my ISP is hugely higher than Comcast as well [3], with Comcast's Net Promoter Score actually being negative. [4]
Those are not signs of a competitive market, and if the market isn't competitive, I think it's a 101-level mistake to say that the price people pay is the true market price.
[1] https://www.cnet.com/news/comcast-offering-1-gigabit-per-sec...
[2] https://www.vice.com/en_us/article/7xgne9/locally-run-isps-o...
[3] https://www.sonic.com/care
[4] https://npsbenchmarks.com/blog/best-internet-service-provide...