Because when you have a lot of money, you can loan money at really low interest rates. Rates that are lower than the rate of return on the investments you'd have to sell to avoid the loan in the first place. If banks are willing to give you really cheap money, you take it.
They also have many many billions sitting in cash accounts with CS earning 0 (and in some account negative rates).
But yes, deals are much easier to structure as 100% debt and the investment, maintains liquidity and limits liability. Set up investment co, borrow with gaurantee from parent etc.