Steem is based on blockchain technology and one of the key points is that it shouldn't rely on any central authority in order to function. If steemit.inc stored passwords then it wouldn't be a permissionless/trustless model.....and so there would not be so much need to use a block chain in the first place:)

There are many advantages of block-chain technology, but the fact that users must be responsible for their own keys/passwords means the end user has to take more care!