Having said this though, it sure seems that software is alot more unreliable than most bridges and buildings, so the question is, "Why?"

That's a good question. Off-hand, I would say that part of the reason has to do with experience. Humans have been constructing bridges and buildings for thousands of years. As such, engineering of this sort is generally well understood. Occasionally, when working with new materials or grander scales, past experience fails, e.g. Tacoma Narrows Bridge.

There is another important way that designing software differs from designing structures. Whether a building stands or collapses is governed by the laws of physics. On the other hand, software design takes place in an entirely abstract world, where the laws that govern it are sometimes subject to change. That combined with the sheer scale of much software makes it difficulty to thoroughly reason about it.