Because encryption hides the data content, whereas you need it in the clear to computationally manipulate it in a useful way, at least with conventional computa

Because encryption hides the data content, whereas you need it in the clear to computationally manipulate it in a useful way, at least with conventional computational frameworks. How is a program going to make a decision at a control flow point if the data it needs to make the decision is encrypted? And if the data's not encrypted, and you're running on untrusted hardware, then it's compromised.