They do, actually. Spamming is big business, as is scamming. Market (financial) incentives are largely at odds with what drives healthy relationships and communities, be they online or offline, because it introduces a conflicting agenda (profit) in people’s behaviour.
One need only look at what a great success (sarcasm) capitalism has been at driving and supporting community values. There’s a reason that pretty much everything related to public service is either served by non-profits or the state: because market economy doesn’t incentivise it.
You can also look into the studies that have been done in behavioural economics that show how financial reward skews behaviour and can even reduce performance. It’s a fascinating field!