You ask some fair questions. I think there are a number of reasons why online dating sites are not as helpful as they can be for people.
1. Expectations - This is a big one. I often compare online dating to a gym or a set of golf clubs. The reason being is that online dating is a tool to help you towards a solution it is not a solution in it of itself. Many of the reasons singles have difficulty finding their spouse will still be an issue regardless of what "tool" they use be it online or otherwise. Singles often make the mistake of thinking the tool is the problem or they don't have enough opportunity when it may be they they are not making the most of what opportunities they have. Let me illustrate using my two analogies.
Online dating is similar to golf clubs in that the clubs are only going to perform as well as the golfer using them. Thus just because a person goes out and purchases the same exact clubs as Tiger Woods does not mean he/she will play like him. Conversely if Tiger woods plays with an old rusty set from the 1950's he is still going to be able to beat 90% of the golfers in the world. His skill as a golfer is what makes him great the clubs while important only make a difference in 5% of his game or less. You can follow this line of thinking with almost any sport, hobby or trade skill. Thus no matter how good an online dating site is the question is still how good of a dater is the person using it. How do they view themselves, how good are their interpersonal skills, how do they view the opposite sex, how much do they understand the purpose of marriage, discerning marriage, purposefully dating for marriage etc. etc. All of these issues an more have little to do with how you meet a person be it online, bumping into their shopping cart or at a bar. The question is how do you handle things once you actually meet them.
Like a gym online dating will only work as well as what you put into it. Most people treat online dating like a Ronco kitchen appliance "set it and forget it." But what are you actually doing when you create a profile of yourself? You are trying to provide an accurate presentation of who you are as a person or at least a start. But I see many people do not put a lot of effort into their profile or if they do they rarely revisit it or regularly upload photos. Let me make a real life analogy. How many times have you heard people say that "When I first met him/her I was not all that into them but I saw them around a couple times and one day I just though maybe he or she would be good to date." Not everyone has love at first site or even like at first site in the real world, so why would people have it online? If you saw a person at a regular gathering and they always wore the same outfit, did their hair there exact same way, wore the same shoes and said the same things what would you think? An online profile is the same way, you have to keep putting yourself out their in different and new ways because the initial profile you set up needs to change and also it certainly captures only a small part of who you are as a person. I don't think it is a stretch to say most people should change something about their profile once a month and photos would be at the top of the list.
The other area is messages. I can't tell you how many users we run across that make one of three common mistakes with messaging. 1. They copy and paste messages. 2. They use a phrase similar to "Hey I like your profile would you check out mine or like to chat?" 3. They share way too much information up front about things that are not appropriate for talking to a complete stranger for the first time. Although you are typing messages you have to think to yourself how would I introduce myself to this person if I met them in real life rather than online. Heck even ask another question would I even introduce myself if I had to do it offline rather than an online dating site? We often see so many people get frustrated that make one or all of these mistakes.
Dating is hard, no question. But I also see so many singles set themselves up for disappointment. While each person is different and each has different issues when it comes to dating I do think singles to not look inward enough. I don't know know how to help you date better, I was a terrible dater before getting married and I made just about every mistake one can make but what I can offer is some general principles based upon what I and our team observe in working with singles every single day. I hope some of this information is helpful.