Ranks are about rating players based on their skill compared to one-another.
Which ultimately means if you aren't that impressive compared to other people you're not going to advance.
It's not an RPG system where you get to advance simply because you showed up.
As for loosing more rank than winning - not 100% sure what the gain/loss difference is due to, but it does automatically create a pyramid tiering of the different ranks, making each additional rank progressively harder to achieve than the last.
Which carries a few advantages.
For one, players aren't going to get a top rank by accident.
Secondly - it makes higher ranks mean something. If ranked was set up more like a rectangle where there were ~10,000 bronze, ~10,000 silver, ~10,000 gold, ~10,000 plat, ~10,000 diamond, and ~10,000 challenger players it would make the higher ranks feel a lot more meh.