What do you mean? Most VC-backed startups sell far more than 50% of their company for far less than 10B.
What would they have done instead? They still need to fund the operation somehow, and raising money is a good way to build partnerships.
If you were sure it will be a $1T business you have more reason to sell equity and accelerate the growth of your company, because you know the remaining 50% is going to be so valuable.