That's a major problem with many (maybe even most) psych studies. I don't think anyone who's thought about it at length really thinks these kinds of populations of convenience ("recruited N students at my university") are a good way to do science, but they are... convenient, so they keep getting used. At some schools, psych students have to participate in a certain number of studies for credit, which makes it easier to get participants, while likely further biasing the participant pool.
It's part of the traditional split between quantitative and qualitative psychologists (and sociologists), each of whom gets one side of the equation stronger. The qualitative side typically goes "out into the world" and tries to interview a target community, building an understanding and iterating to fill in gaps in the understanding. They often succeed in quite good coverage, but don't produce statistical results. The quantitative side typically wants to measure statistically significant results on pre-specified, narrower questions, but often does so only on populations on convenience, rather than going "out into the world" and really understanding the populations that are out there.
A comment I've heard before from a psych researcher is that psych research at universities is the really the research into undergrads taking psych courses...
This is very, very true. In fact, Heinrich et al wrote a wonderful paper decrying this very problem.
humancond.org/_media/papers/weirdest_people.pdf
Unfortunately, its not going to change, because of the pressures on academics to publish, publish, publish inclines them to take full advantage of these samples of convenience.
Its actually even worse, because the majority of psych papers are based on American psych students, so there's not even much of a good geographic spread in the ridiculously limited samples psychologists use.
It's part of the traditional split between quantitative and qualitative psychologists (and sociologists), each of whom gets one side of the equation stronger. The qualitative side typically goes "out into the world" and tries to interview a target community, building an understanding and iterating to fill in gaps in the understanding. They often succeed in quite good coverage, but don't produce statistical results. The quantitative side typically wants to measure statistically significant results on pre-specified, narrower questions, but often does so only on populations on convenience, rather than going "out into the world" and really understanding the populations that are out there.