That is a good question; I assume the answer is that they cannot get a 40B valuation if they IPO.
People know that ARM is used in all smartphones and after Apples M1 and the growing usage in Servers many believe that ARM is the future.
Also - with an IPO, customers of ARM like Apple could easily buy shares of ARM...
Probably the reason why Apple didn't bought ARM is because they know they wouldn't get through the competition authorities.
But with an IPO, Apple and other customers could buy something like 9 percent of ARM...
However they probably cannot sell all the shares at an IPO and get the full 40B in cash which maybe is what SoftBank wants at present.