The landlords depicted in the book are there to make money out of their tenants. They don't see themselves as providers of charity, or as glorified social workers. They provide accommodation to those who need it, but at a price. Money is the most important consideration for them.
The housing market is so broken that the likes of Sherrena and Tobin can pretty much do as they please. They can continue to provide substandard accommodation, safe in the knowledge that there will always be tenants desperate enough for a roof over their heads, such is the chronic shortage of housing in Milwaukee.
Landlords could always improve the lives of their tenants by carrying out vital repairs, or, at least, making an effort to make their accommodation more habitable. As there are no financial incentives for them to do so, they make no effort to improve their tenants' lives. In this neck of the woods providing squalid accommodation pays, and so long as it does, the likes of Sherrena and Tobin will continue to cash in.
Sherrena and Tobin could also have been more fair to their tenants by being more flexible over rent arrears, but they choose not to for the reasons already given. They know full well that there are dozens of potential tenants who can take the place of those currently falling behind on their rent. So it's simply not worth the hassle to show more flexibility.