That is false. Plenty of people build death traps as their own home. Nobody will know if you remove a load bearing wall without a permit so long as you are not obvious about that work. Slumlords are generally in a place where people will know.

Slumlords do want cheap, but they need to get money from the bank and that means the bank will care about quality. Slumlords also don't want to lose everything if the building burns. Note however that quality to a bank and slumlord are things you cannot see and so their buildings often look dangerous while being safe. (I'm not saying all slumlords are perfect, just that their reputation is on a few graphic failures that often were 100 years ago)

The permit isn't what keeps you safe (except from being shot by the government at the behest of people who think like you). What keeps you safe is not doing a shit job. When working in one's own home it's far easier to just over-build things than to engineer things to the bare minimum or pay someone to tell you what that minimum is.

Also slumlords are the guys doubling the number of units in an existing triple decker, not the guys building new stuff.