It's in the name. Why not name themselves ModelRouter or something similar?

If they lead the market, they'll extract value in lots of ways that an open company could at least be compelled not to. Plus there won't be competition.

They're probably selling your data to LLM companies and you don't even see what they're doing.

Without competition, they'll raise their rates.

If they were open, you could potentially run the offering on-prem. You could bolt on new providers or use it internally for your own routing.

Lots of reasons.