A major drawback of probability sampling is that it can be time consuming. Probability sampling is a sampling technique in which samples are gathered in such a way that the entire population has equal chances at being selected.
Having a truly random sampling dictates that you must set things up in a way that ensures the most random results. Throughout history human have practiced different types of random selection. This has included things like pulling a name out of a hat or choosing straws to see who comes up with the shortest one. This type of random selection is often done with a computer these days.