I am not the OP, but here is my take. In practical sense, because they do more harm than good. In logical sense, IMO, IP (patents, copy rights etc.) is not `property`, so it cannot be owned. Thus it can be copied indefinitely and used once it is exposed. World would be a better place without this nonsense.

They serve unintended purposes better than their intended purpose.

There’s something better and no one cares to try.

1. The software patent examination process is pretty broken - a lot of companies try to patent everything without regard to is it obvious or innovative so if they get sued for patent violations they can counter sue; the patent examiners for whatever reason (law, lack of expertise, lack of staffing) keep approving patents for software that is obvious not-innovate normal state of the art things) then all of a sudden people can't use linked lists or whatever.

2. The idea of patenting algorithms themselves seems to fly in the face of free speech. An algorithm is a disembodied mathematical thing, and not a device for selling. The same logic that allowed PGP to be exported as a book despite export laws would seem to apply to patens on algorithms.

3. An industry that is harmful to the free development and operation of business has sprung up where firms (that don't do anything economically or socially productive) buy a lot of patents up and then file nuisance law suits to get revenue streams for their investors.

(Disclaimer I am the inventor on a couple patents; I had prepared my notes and list of innovations on the systems involved for the meeting with the lawyer filing the patent, and the discussion never got to any of the innovations involved, so there is no way that the patents were awarded for innovation.)

They hold back progress and prevent things like h265 being adopted