I agree with you on most of the things you said...
There are a lot of minnows here that are trying to fight their way up. Comments are marketing to promote yourself on this blockchain. This is the only thing you got, without it, you can write the best post on platform and it won't be noticed.
That said, your comments are your ID here and that's why some people are trying to write a very good comment. That's also why some people don't want to write a small comment because they think that that will be considered as a spam or a very bad comment. That means that your rep falls down in other people's eyes because of that small comment.
It is easy if people know you here already or if you have a lot of SP, then you don't have to write meaningful comments and you can afford to have fun and writr whatever you like.
Other thing is that if you write a good comment you will probably be replied and engaged in conversation. With a small comment you will just be in the bottom of the post.
About upvoting your comment... I did it only once in my STEEM career xD I understand why people do it, but I don't agree if you're going to upvote your shitty comment. If you are going to write a good comment then it's ok go upvote it so that it doesn't get lost in the sea of small comments.
It happened to me a lot of times that I write a big meaningful comment but I wrote it 2 days after the post was made and author didn't catch the comment because it was at the bottom of the post. I hate when that happens and that's why I partialy understand why people upvote their comments.
I believe these are the reasons why engagement is not that big. It is expected from minnows to write a good big comments and that is sometimes tireing.
I remember when I did this. It was awful because sometimes you just wanna say Haha that's funny, but instead you need to write abig comment and explain why it is funny. There are too many spammers here and if people don't know you around here then your small comments can be easily seen as spam comments...
I didn't like that way because it made me to start writing semi generic comments. Now, I write a good comment when I want and a silly one when its needed :))