At our current level of software technology and accessible computing power, computers can't do a whole design.
Computers can automate some parts of the design process that can be well specified in a formal sense. For example, once you specify what pads have to be connected to what other pads on a PC board, minimum trace/width spacing and other rules, software can often do the actual routing of tracks.
Even this is far from fire-and-forget with current technology, but the big issue is that this is only a small part of designing a electronic device. Humans still have to decide what the device is supposed to do. Nailing down the specs is a important part of designing something, and these kind of constraints are hard to codify in a machine-understandable way. There are just too many possibilities such that software that understood all but dumbed down and contrived specifications is beyond our reach today.
Another part that would be very difficult for software to do is the actual creative engineering design part. There are a vast number of possibilities, each with its own set of tradeoffs. A computer could possibly verify that a particular design meets a particular set of specs (although, again, codifying those specs for anything but toy problems is too complex currently), but finding the optimal or reasonably optimal design is much much harder. That's where the creativity, knowledge, and experience of the engineer comes in. It will probably be quite a while yet before computers can mimic that for broad classes of real world problems.