I do believe that your evaluation has some merit. It is difficult to win most of the scenarios the first time through.
Why do people lose so much? Probably because they don't read the scenario description and the briefing. Having an understanding of your objective, your equipment, and that your opponent is essential to winning.
Additionally, as you indicated, scenarios are often over-built. The scenario designers often use the best equipment available, and they often load the scenario with lots of it. Are they realistic? Probably not. But, scenarios that are actually realistic can be boring. An example can be, "find this task force/sub and sink it." Well, you search all over the place and it's nowhere to be found. I can think of a few scenarios like that. If there weren't other "things" to do in the scenario, it could get really boring.
Sometimes the scenario designer will throw you off by giving you spurious information. Sometimes they will take you by the hand and lead you directly to the task. The tutorials are like that.
Some scenarios even throw in some "side missions" to occupy some "dead space" such as "rescue this hostage" or "pick up a downed pilot." These are filler items, and they are designed to keep you busy while other things are being set up.
I've found that most scenario designers are really good, and that they will give you plenty to do. As I said earlier, I also believe that if you spend more time reading about the scenario, and the briefing, and the objective, then you will have a lot more success. But, sadly, most just want to jump in and play, and they don't fully understand the objective, or know their adversary, let alone their own equipment.
I look at each new scenario as being a Christmas present. It's a surprise. You can only unwrap it once. After the paper is off, the surprise has been revealed. So, enjoy what is being placed in front of you. Take in all that the scenario has to offer. Enjoy the moment, and don't worry about "winning" because, as you said, it's not likely going to happen.
I particularly love the scenarios that are randomized. By that I mean, they can be different each time. The objective may be the same, but the number of units and where there are, or even if they are present, is always in question. This makes the scenario timeless, and it's fun each time that you play it. (I do that for each scenario that I create. I create them for myself, therefore I need them to change so that I don't ever get bored because I "know" what is going to happen.)
Many scenario designers make the scenario challenging, because if they didn't, it wouldn't be much fun.
If you want to "win", then I suggest that you READ everything BEFORE you begin play. Every word written in the scenario description and the briefing is there for a reason. The scenario designers do a marvelous job, and I applaud their efforts. As I said, each scenario, to many anyway, is like a Christmas present. You can definitely better your score if you READ the briefing and follow the storyline. Without fully understanding the scenario and its objective, and just beginning to play without reading it, you are just shooting into the dark and hoping for the best.