Why patents are not cited in papers as that of journal papers? I couldn't find papers that cite patents. Is it OK to cite patents? If yes, what are the reasons for scarce patent citations?
There are certainly papers citing patents, mostly in engineering fields. So, yes it is certainly OK to cite a patent. 

Now patents have an entirely different purpose than scientific articles. They usually contain detailed descriptions of methods and devices and (sometimes wild) speculations about practical applications of these. Useful to protect potential commercial use, but not really to scientists wanting to build on existing knowledge.

More specifically, what they typically don't contain are original experimental data, or other means of scientifically evaluating the described methods or devices.

Very often, if the development of the patented method or device has led to scientific discoveries, these have been published separately as articles (or conference proceedings for the fields where that applies). Those will be much more scientifically informative, and hence more likely to be cited in academic articles.