Everyone writes the same stuff. Most profiles are simply identi-kit personalities. Among the girls at least the people being described are like stock characters from the world's most generic romantic comedy. I'm fun, quirky and outgoing. I love my life, I'm in love with life, I love the life I live and live the life I love. I like hiking, wine, and crossfit. I like going out to bars, but also kicking back on the couch watching Netflix. Friends and family come first. No hookups!
Lest you suspect that I'm just making fun of the women here, there's very likely male equivalents. The beta version is 'I'm a laid-back sweet funny guy who likes restaurants, movies, going out, staying in.' The jock alpha tool version is '6'4, 220 llbs, I'm just on here looking for young girls who are up for for some fun.'
You may think is that this is an explicit form of herding - there's a certain meme or profile idea that people are referencing, perhaps, or that people are trying to signal that they're of a certain type and thus tend to get bunched together with others of that type.
This is possible, but one big factor militates against this being likely.
To wit, most people never actually look at many (if any) profiles from members of the same sex as them. They're writing the same thing, but they most likely don't realise that they're writing the same thing. (Incidentally, this is why I have more familiarity with what's common across female profiles than across male profiles).
As far as I can tell, there are two ways to interpret this.
The first is that people are all fundamentally the same. They work similar crappy jobs that they don't feel define them as people and hence they don't really want to talk about. They relax by drinking beer, watching sports and going to the movies. Some people vaguely feel guilty about this and think they should be doing stuff like reading, cooking and hiking, so that often makes it on the list as an aspirational description, but really most people have no interesting hobbies, nothing they're particularly passionate about, and no unusual interests. And it shows.
Don't get me wrong, there's certainly a significant element of truth to this. But I don't think that's all that's going on.
The other possibility is that people are simply bad at describing themselves in ways that would be useful to others. A similar basic claim would also explain the Dove Beauty Sketches nonsense that the Last Psychiatrist talked about, where a guy draws a sketch based on women's descriptions of themselves versus a sketch based on strangers descriptions of the same women, and hey presto, the stranger is more accurate. Their punchline is that everyone is actually beautiful. I'd say that people just don't know themselves very much.
Even among the population of identically described beer drinking, football watching, bar attending members of the opposite sex, it probably wouldn't take too many minutes of conversation for me to work out whether their personality would be conducive to sitting through a whole dinner with them.
Of course, much of that useful variation comes from things that people may not want to put in their profiles: 'I'll tell stories that go on forever without an ability to read that you're not interested' 'I'll give off a vibe of self-centeredness in the stories I tell about my interactions with other people.' 'I won't have anything interesting to talk to you about'. The last point, of course, sounds like the first theory, so they're not totally disconnected.
But even so, there is some useful information that could be given that is appealing to the opposite sex, but people still don't know how to describe it.
Sometimes, the stuff that's true and flattering may still sound weird to describe. 'I have an appealing way of smiling and maintaining eye contact while we talk'. 'I'm not jealous if you want to spend time with your friends.' 'If we end up in a relationship, I'll leave sweet notes and cupcakes for you in the morning sometimes just because I was thinking about you.' 'I don't hold grudges for very long.'
The lowest level of introspection is to just answer 'What's a flattering but true description of me as a person?', or 'What do I enjoy doing?'. That way leads to drowning in cliche.
The next level of introspection is to think about 'What attributes of me as a person can I talk about that will actually be appealing to the person of the opposite sex?'. If you're a guy writing of your love for watching mixed martial arts, or a girl talking about how she owns multiple cats, these traits may be true, but they're unlikely to be well-calculated for appealing to the likely interests of the other person. Why not start by describing things that they might like about you, instead of just things that you like about yourself?
The highest level is to ponder the question 'What attributes about me will be appealing to the opposite sex and set me apart from the zillions of other profiles that the person is most likely reading?
Which gets me to my overall advice on how to write one of these profiles. Write a draft profile that you think might be vaguely appealing. Then go through whatever site you're using and read a whole lot of profiles of people from the same sex as you. Look at what kind of cliches and boring phrases keep cropping up. Go back your draft profile and delete every single one of them. Then write only about the things that you haven't seen over and over, or the things that seemed neat in other people's profiles.