(1) Judging attraction from a profile is very difficult, so you can go spend a lot of time and energy going on dates with someone who is obviously wrong for you.

(2) Many, perhaps most people, misrepresent themselves in their profiles, making "shopping" all the harder. In my experience this is true of women, but women say that men lie too.

Attraction is much easier to sense in person. Once you're attracted, the answers to a bunch of fact-based questions don't matter so much.

There's a massively skewed ratio of men to women on these sites, probably around 10:1 men to women. As a result, women get spammed with no-content or offensive messages, driving many away. Those who do stay have to spend significant effort sifting through messages to find good ones, and inevitably a lot of good messages from decent men get lost in the deluge. Most dating sites are just glorified personal ads, no innovation at all. OKCupid is an exception with its quizzes and match percentages, but the percentages are inaccurate and OKC is filled with people who are just there for the quizzes.

As other people said: for most sites the model is just broken.

The website needs a large amount of potential matches on the site. So, once you sign up your profile is never removed. That means people looking sometimes have to trawl through dead profiles.

You earn money by charging people money. Sometimes that's for reading messages people send you. Sometimes it's for replying to messages. Many sites send fake messages.

Many sites are run by a small number of operators and they buy and sell profile information among themselves. This would be unacceptable if the profiles were genuine, but they're often fake. So there are huge amounts of fake people sending fake messages to real people on websites just to get them to pay to subscribe.

But even the good sites have problems. See, for example, OkCupid. (which is a very good site.) There's a small number of people on there who have little intention of actually dating. They're just meeting online people. OKC have taken steps to reduce that (really, they should have opened up a pay-for forums, a bit like suicidegirls. SG had "come for the titties, stay for the forums", OKC could have "Come for the dating, stay for the community". (Yes, I'd like a free sub if you ever do that, OKC)

Or people just have wildly unrealistic expectations.

And, on top of all that, you have the regular suckiness of dating.