Why SBMM ''cuts out the middle man''

I think you're right that the "middle man" is the victim here. I'm a below average player myself.

I have played around 3700 games right now, I have around 2000 lifetime kills, Once (really once!) I got 2050 damage with Wraith, so I have the 2k Badge for Wraith.

My max kills in one game is 8. I think I got this around 3 times (in this 3700 games). My lifetime K/D is 0.55, in season 3 I ended on 0.79 and now it"s 0.97 in season 4, so I do make (a little) progression. My level is 195 now. I've got around 100 lifetime wins.

Yet when I play unranked, I see at least 3 predator contrails per game. 

I read somewhere that only 0.2% of the players is a predator. How is it possible then that I get matched up with these predators almost every time??

My highest rank is Gold 1 so far. I think I can make it to Plat, but I try to avoid that because I think I will be destroyed there almost every time.

I think the system gives a lot of value to your level and the total amount of games played. Also It feels like the opposition is harder when I play with my (pretty equally "skilled") friends in a premade squad.

I'm for a good sbmm system, but this system is not working good enough. It seems that there are only 2 splits: or you're  considered "bad" or you're considered "good", and in the second case you're matched up against the predators.

Also another reason for the bad functioning of sbmm can simply be because there are to few players online, so the player base got to low and so the game cannot divide the players into the correct "category"?

I still love this game but it could be even better when sbmm would work as it should: put players of (almost) equal skill in the same game.