Because that’s how the math works. You lost to people of a certain rating, so appropriate rating was subtracted.
Elo is a measure of your performance and assuming a constant skill level, it is normal for it to oscillate 200-300 points around a mean the long-term, based on factors like luck with comps, tiredness and so on.
Because then it would not take into account the skill level of the players. It would not be fair if Magnus Carlsen could farm chess rating off of 7 year old kids now would it. And if you wanted to queue only players within some certain delta rating of eachother, say only 1990-2010 allowed in the lobby, well that would make the queues insanely longer.
If you did not impose this restriction - imagine you get into a lobby with players 300 rating higher than you on average. Assuming the ratings are representative, you would not stand a chance, maybe sneak in one lucky win. Would you really deserve to lose 6*7 rating in such a scenario? The player experience would be much worse in such a system.
The system has evolved to the current state from the traditional elo system (introduced in TBC) because it makes it so high-level players don’t have to stomp through the low-level players on their way to the top, thereby making the experience better for the low-level players and also it makes it more difficult to exploit the system.
People keep getting mad at this system because they do not understand how MMR works. If you go 0-6 against people of a similar skill level, you lose 70 rating. Now, you lose even more MMR. So, now you have around 100 MMR less than before. BUT, given the matchmaking algorithm, it is entirely possible that you enter a lobby which is being played at even LESS MMR - maybe another 100. On average, ignoring compositions and such, you should certainly have a high win rate in such a lobby, as effectively you are playing 200 rating below your mean. It is expected that you should win most of those games, and therefore a 6-0 won’t be rewarded with a 70. Now, should you enter a lobby which is 100 MMR higher than yours, it is not expected that you should win most of those games and the reward for a 6-0 would be worth more than 70.
As for the solo shuffle format and its fairness - that is is another question. Naturally there are going to be some lobbies that are going to favor certain classes and disadvantage others. The idea is that over a large number of games, that is going to average out. Which can be annoying with such long queue times.