People say this about AWS/GCP/Azure. They are so willing to compete on cost on everything else that it seems odd like this would be true, so I have a different theory:
OVH/etc are all able to massively oversell bandwidth. Most customers do not use it. This is fine, and not anything I think they're doing wrong!
But the "Big 3" are all about automatically scaling resources. With my own applications, I find that if I am scaling up/out, my network traffic is growing at a commensurate rate. Whereas back in the day when I was using SoftLayer or other places where I leased dedicated servers, I was not using anywhere near the included bandwidth.
It's easy to sell 1TB at the price of 100GB if people, on average, are only using 100GB. But then you have to sell 1TB at the price of 1TB if people, on average, are using that 1TB.