Jet engine design is iterative, and the basic principles are well-understood.

Drug design seems a lot more binary. You can find a new pathway, but drugs themselves are fairly simply molecules, and you can't iteratively 'fix bugs' the way you can in an engine or a piece of software.

The engine is a given, it's almost astronomically complicated, you know a lot less about how it works than you'd like to, and you're trying to change how it works while it's running without breaking anything, using tiny rigid parts that have to snap into place correctly and can't be bent to fit.

High failure rates aren't surprising.