You're right that companies diversify into other asset classes, but cash is still a major asset class and that risk needs to be managed.

In contrast, Apple and Qualcomm sell product all over the globe and are currency diversified by default. Softbank revenue on the other hand is overwhelmingly in yen, hence the vast diversification into foreign companies. Since they don't export much they also miss out on higher margins internationally if the yen were to fall.

Also, the BOJ's negative rate policy allows SoftBank to raise cash cheaply with uncommonly low yield A-rated bonds, effectively subsidizing their foreign buying spree.