I am not a data person. So I can only provide insights into the theoretical incentives behind the phenomenon. Moreover I think that there are multiple forms of such web spam and each is rooted in distinct incentives. Hence, there is not a single true answer.
Let me first express my view on classical (email) spam and relate it to what you have in mind. I will do this in this answer by using a baby model any undergrad could come up with. Then, I will point to search diversion incentives of an intermediary in a second answer building on the model by Hagiu and Jullien (RAND 2011). You (and the community) can then choose what fits best.
I guess the classical incentives behind (email) spam arise because the marginal cost $c$ is vanishingly small while benefits $b$ are comparatively large, $c \approx 0, b>c$. Although only extremely few people click the spam link, it can pay off to send a ridiculous amount of emails, because some do click and thereby end up on the homepage.
The spammer maximizes $(b-c)q$ which is strictly increasing in $q$, the quantity of emails sent. Hence, she chooses the maximal possible amount $q$, i.e., send the spam to any address available.
The benefit $b$ here is that each visitor to the homepage may buy a good or generate ad revenue. In the first case, $b=p_1 p_2 V$, with $p_1>0$ being the probability of the average email reader actually clicking the link and $p_2$ the probabiliry of buying a (possibly fraudulent) good from the spammer (or transferring money to the Nigerian prince) and $V>>0$ being a large profit from this transaction. Since $c \approx 0$, spam pays even for extremely low probabilities $p=p_1p_2$.
Alternatively, the spammer is just an intermediary that leads possible customers to a seller's homepage. This intermediary then gets paid either per visit or per transaction.
Both lead to the same model as above. The seller would be willing to pay the spammer some fixed amount $x$ per visitor with $x \in [0,p_2V]$ such that spammer and seller can agree on some spam benefit $b \in (c,pV]$, and the above model applies. If the seller pays some share $s$ of $pV$ per transaction, we can also arrive at the same model with $b=spV>c$.
Alternatively, the link just leads to a homepage full of ads. Then, the spammer gets paid per click on the ad or per second the ad is watched. Both is isomorphic to the model above with additional middlemen. Another alternative is that the link downloads malware, resulting in the same model with $V$ being the value from successful infiltration of the malware and $p$ being the probability of a successful undetected download.
Instead of $q$ being a quantity of emails sent it could be an additional keyword for search engines. Since adding one more keyword is quasi costless and attracts an additional number of expected visitors $r>0$, we arrive at a similar model with $(rb-c)q$.
This baby model abstracted away from a lot, but I think this is crucial incentives are rooted in very low marginal cost and the law of large numbers. Of course spam bothers people and engaging in it comes with a loss of reputation. For that reason, you do not see established firms doing it.