There is complexity in all areas of engineering of course. What makes software special is that it is so easy to create new software, and thus create more complex things.
In fact it is more difficult to create software with simple design than it is to create complicated software. A "hacked" thing becomes very complicated as it evolves. Software is isomorphic to logical proofs, we can think of how it is often possible to simplify mathematical proofs. It is a lot of work however to come up with simple elegant proofs. So it is with programs.
Mechanical devices have to be carefully designed because idea is that once you have the design you will then manufacture many machines having the same design. You need to do that because manufacturing physical devices is expensive. In software you can just start typing.
In fact it is more difficult to create software with simple design than it is to create complicated software. A "hacked" thing becomes very complicated as it evolves. Software is isomorphic to logical proofs, we can think of how it is often possible to simplify mathematical proofs. It is a lot of work however to come up with simple elegant proofs. So it is with programs.
Mechanical devices have to be carefully designed because idea is that once you have the design you will then manufacture many machines having the same design. You need to do that because manufacturing physical devices is expensive. In software you can just start typing.