The reason why penalties still apply for people who leave (or crash/disconnect) causing the match to shut down is to prevent any incentive for queue dodging. For example, there may be a specific map that you do not like or a particular player that you do not want to play with, attempting to dodge it by disconnecting would be considered cheating and disrupts the overall experience for all 12 players.
Furthermore, you are expected to play Competitive with a reliable system and connection. While crashing or disconnecting can happen to anyone at anytime, if you are repeatedly disconnecting or crashing and not taking the time to identify and correct any issues, that makes you just as responsible as any person who leaves deliberately. This is why the starting penalty is only 15 minutes but quickly ramps up for those who do not take such problems seriously.
More details on this policy can be found here: