They are receiving $19B in annual revenue today by having Google as default search engine (which is all profit for them).
Or they could spend billions on the acquisition of Bing + more billions annually to run Bing.
Spending huge sums of money. Or receiving huge sums of money. Which would you prefer?
No, that's not the way too look at it. Apple spending billions to buy the Bing search engine would then let Apple collect 100% of the search ads revenue instead of splitting someone else's ad revenue with a middleman (aka Google).
So the business decision matrix is based on whether 100% of ad revenue minus the acquisition & ongoing maintenance costs of Bing -- would be worth more than X% of Google's revenue split.
Acquiring the "Bing search engine" isn't just the index ranking algorithm. It also includes getting the whole back-office infrastructure of ads-monetization management & dashboards for advertisers to log in to bid+buy ads.
Google is apple's main competitor and building an actual rival might be extremely beneficial for them.
Google supposedly makes more money from this arrangement then the spend, which means of apple can run a successful search engine they can take more of the profit.
Apple has loads of cash which they look for ways to invest, Google shows that search engine can be an immensely valuable investment.
Obviously there are other concerns, chief being consumer sentiment here I imagine.