Help understandign my LP gain/loss

If this is the wrong board, let me know and i'll move it, but I need help understanding my lp gain and loss here. I'm currently sitting at S1 with 96 LP. (lost gold promos a few times) I've always been told that if your mmr is higher than you're league (playing against people of higher leagues/devision) you will gain more for a win than you lose for a loss, and vice versa. So can someone explain to me why I am playing against high gold and low plat elo while gaining only around 18-20 per win and the same for a loss? For example, the game I just played had me, another silver, and 3 gold 3+ while the enemy had 1 silver, 3 gold 3+ and 1 plat 5. We won and I received only 19 LP for the win, and I don't understand why i'm getting such a low amount, making it very hard for me to climb since I'm getting LP gain of a normal mmr player but playing against much higher leagues. No, I am not just playing against people with a VERY low mmr in gold, as I would never be matched against a plat player that has managed to stay in plat with that low of an mmr, so someone please help me understand this, and also answer this question; Why even have mmr? Why not just gain/lose LP based beating someone higher or losing to someone lower than you? If the only answer you can give is so that they can control smurf accounts, I don't accept that as a good enough answer. I don't see why they need to balance a HUGE part of the game around less than 10% of the playerbase (besides game balance around lcs of course) Please and thank you for any answers