OpenAI and Anthropic are trying to pay off a half trillion dollars of investment. They also have the most demand right now, to the point that Anthropic sometimes doesn't have enough compute and that means more limits. They can't stop taking new customers, though, because the market would hate it.

An open weight inference provider only needs to pay for GPUs, or discounted APIs from 3rd party vendors. Same basic financial model but they didn't spend a trillion dollars so their loss isn't as high so they can afford to do more inference for less money, and their demand isn't as high so there's more than enough compute.

Because it offers cheap open source models, not GPT and Claude. I mentioned it as an alternative to Z.ai's subscription in OP's comment, not to Codex.