Because, that's what SPAM is intended to do, only a fraction of a percentage of the people have to give them money (even if it's as a joke, opposite research or any reason whatsoever) for them to be profitable.
These sites are literally auto-generated for any field you can think of (I work in association with physicists, biologists and neurologists, they have at least a dozen journals across these fields). I get daily spams from at least 5 of them. The websites are identical (replacing the $field), the journals look identical and they're auto-generated. It is potentially a one-person operation having these half-wit professors publishing close to $3000/month/journal + advertising pages for what is an entirely electronic "journal".
There are similar sites offering help writing your papers, offering help getting NIH/NSF/whatever funding. A small team could easily lift close to $1M/month in a self-enclosed, self-propagating ecosystem of 'products and services'; everything from the creation to the publishing of an entire study.
The problem is not necessarily that these people are doing that (after all, it's a great idea to create an entirely fake ecosystem), the problem is that our public resources (in the form of government and tuition sourced grant money) are being used to publish these professors in fake, non-peer reviewed journals. Universities and government institutions actually accept these fake journals as 'credentials' because being published in a number of official-sounding journals trumps quality research for the beancounters.