A patent is an absolute monopoly; everyone is forbidden to use the patented process, even those who reinvent it independently. This policy implicitly assumes that inventions are rare and precious, since only in those circumstances is it beneficial.
The field of software is one of constant reinvention; as some people say, programmers throw away more "inventions" each week than other people develop in a year. And the comparative ease of designing large software systems makes it easy for many people to do work in the field. A programmer solves many problems in developing each program. These solutions are likely to be reinvented frequently as other programmers tackle similar problems.
The prevalence of independent reinvention negates the usual purpose of patents. Patents are intended to encourage inventions and, above all, the disclosure of inventions. If a technique will be reinvented frequently, there is no need to encourage more people to invent it; since some of the developers will choose to publish it (if publication is merited), there is no point in encouraging a particular inventor to publish it--not at the cost of inhibiting use of the technique.