Their MO is to offer lots of money at inflated valuation vs domestic investors. This is compelling for founders - lots of money to grow, for less dilution.
That said, there's very little value Softbank adds other than the money. No connections, no advice, and it's generally not a helpful long term partnership. They also don't seem to conduct the level of scrutiny that other investors do, because they have so much cash and want to muscle into hot deals. And possibly also because founders wouldn't want to deal with their scrutiny vs domestic options.
Ultimately you take the money when you are greedy or don't have other good options. And neither is a good signal.
I think Sam is driving as aggressively as he can, given AI seems like a winner takes all type market. Domestic investors are balking at the exponential increase in needed investment amounts given economic uncertainty and lack of justified return. Meanwhile Softbank has been catching up and has been dying to get in on OpenAI. So here's the opportunity.
Maybe this works out and OpenAI is going to land this. But more likely, OpenAI is acting like the music is running out soon and they're throwing a hail mary. And Softbank's limited partners are going to be left holding the bag.