Can't I just do this myself, or maybe buy software or go online?
By all means, you certainly can, just as you can repair your own car, replace your own roof, or even set your own broken arm. You may not save time, but you'll probably save money, at least in the short run. The trouble is that if there's a problem, it might not surface until it's too late. That's when the value of having a professional becomes clear.
The cost of an estate plan that is not done correctly can be huge when compared to the cost of working with an experienced attorney to make sure the job is done properly the first time.