I have to agree, and have been wondering along the same lines. I went on a 5 game win streak, got promoted, then immediately went into a loss streak. 4 losses in a row, ended up back where I started in LP. This has happened on numerous occasions, with sometimes loss streaks lasting 5-10 games over several days (I normally take a break after two losses).
On this most recent repetition, every game went the same way. All lanes lost and when/ if I got ahead in lane i'd get camped. Valid strategy, if the other two lanes are winning, camp the lane that isn't.
I'm far from pro, admittedly. It took me longer than I'd care to admit to climb out up from the division i've found myself in the last two season. But I see this with startling regularity in ranked. Win streak with teams that seem to play well together, then lose streak with flamers/ feeders/ trolls or just people having really bad games (myself included).
Worst of all, after said win streak, I got my secondary role 3 times in row with 3 losses. At one point this season, I had close to a 55% win rate, the best i've ever attained. Before this season I'd never had more wins than losses. However, due to loss streaks like the above, i'm now sititng at 51.5%. I will say, however that my winrate in my main role is closer to 60%.
All in all, if nothing else I'm glad to see i'm not alone in feeling this way. It may simply be RNG, extraordinary swings in fortune, or simply getting tilted and every game subsequent feeling hopeless. But game after game where the enemy team is 10+ kills ahead within the first 10-15 minutes makes it hard to not get disheartened.
Probably just time for another break from ranked.