Why should I lose LP when a teammate dc or goes afk?
Because, if you didn't lose LP, the enemy team couldn't gain LP, otherwise there'd be LP created out of thin air, which can cause large issues further on.
Moreover- if me and my team win a ranked in 4v5 it would be fair winning more lp
That'd be fair. If you win the 4v5 the ally still loses LP, so your team gaining what he lost would actually help fix the ratio.
BONUS: WHY THE FUCK SHOULD I CARRY A TEAM OF APES AFTER A WINNING STREAK?? What does it mean that if I'm winning matches with and against my pairs then I will be matched against strong teams but very weak teammates?
Because you can't get good teammates all the time? Win streaks are likely because you got teammates that didn't escape from the zoo. But someone has to get the baboons, and sometimes that someone has to be you.
I really hate this fucking matching system, not fair at all
Neither of these are matchmaking issues. The first one has nothing to do with matchmaking and is just how Riot awards LP. The second might be able to be argued to be matchmaking, but it's less matchmaking and more people just having crap games and you having the misfortune of having them then.