There can be many reasons to downvote without a comment.
The first and most important: sometimes you don't have the nerves to explain to the OP, without being considered an insult. For example, if it is clearly visible, that the OP could write correctly, but he intentionally doesn't even try to. Saying to him, where should he go with this attitude, would violate the Code of Conduct. But silently voting him down is okay. It might have a psychological effect, too: by allowing him to think about, why he could have got the down, could give him ultimately a much stronger motive to improve, as a flamewar.
However, not this is the typical problem.
The main problem is that many people, as I understood, have the habit of shoot his daily 40 "rockets" to questions. They are doing it every day.
It is very visible, because using the site as it is intended - searching for answers to our questions - simply doesn't lead us so many bad posts. The only way what I can imagine, to have many times more downs than ups, if one intentionally looks for downvotable posts.
I don't know, why are they doing it. Maybe a psychologist could decode them, or at least they could give some help to accelerate your own research.
My current best hyphothese is that they are executing some gamification algorithm, i.e. they are essentially playing a game, where their goals are very different from the goals of the voting system (what is to separate bad posts from the good ones, helping the search/indexing system to give good posts to the visitors). For example, they might want to decrease the count of happy programmers of the world. Or they might vote their friends up, everybody other down (sect mentality). Obviously no one explained it clearly, but searching for "rep farming" in newer, and for "rep whore" in older meta posts, could reveal at least their "explanations".
I checked your SO activity, and my opinion is that your posts are okay, and your skills make you a worthy contributor. You clearly don't deserve the downs. However, you don't get too much. Well, I would suggest to formulate your titles as round questions, instead of your current sentence fragments.
A possible symptomatic handling of the problem, if you try to compensate them: give ups easily, and give downs only to the real crap. It won't help you, but you can make the site a more friendly one.
Another possible workaround is using secondary account for your questions.
If you have a deeply and unfairly downvoted question, you can also ask the team to detach it from your account. I personally never did it, but all those who did, said that it happened seamlessly.
My answers to your other sub-questions:
Why SO/SE still has the feature of anonymous downvote without explanation. When there is a flag option what is the benefit of having downvote at all?
The SE f*ly needs your activities. They need you. They need your votes, they need your comments, and most heavily, they need your answers. They are using the votes to show their best face to the google, and to the visitors. If they would harden voting, they would have much lesser information to tune their indexing.
What is the benefit of having so many SE forums when SO has tags? How flagging a post as off-topic and closing/removing it helps SO community?
Why there is no option to move questions across these SO/SE forums if being "on-topic" is so important?
All the SE sites should run from a common database, with loosely coupled tag system. As I've first found a different SE site after the SO, what was the ServerFault, I've thought that the questions are coming from the same database. Only they are somehow filtered for ServerFault topics and not for SO topics. It was simply so obvious, that so should the site network work. And my this opinion did not change since then.
However, the SE works on a much more... well, say, simpler way. The SE sites are entirely different databases, with zero connection to each other. Later some message passing system was invented, what was a imho a very bad idea. Instead, the databases should have been unified.
The SE probably has not the developer resource, and also not the intention, to re-develop their site interaction mechanism correctly (with unifying the databases). This is why the question migrations are so hard. They should be so easy than a re-tagging.
Currently, most of the off-topic closed questions are left to oblivion, despite that they could be used to improve their correct site (or to build them together to entirely new sites). I have simply no idea, why the SE gives up this huge power. I have simply no idea. In my opinion, the SE has simply shot himself on the foot with it. My best hyphothese is a big company syndrome, i.e. they might have some internal social inertia, maybe some boss once decided so, and since then no one has the courage to change this bad decision.
They have also a roughy public, although not very advertised explanation. And it is that they want to separate the communities as they can, to protect them from each other.