If spam senders were economically rational actors, then they would either try and generate more leads or improve their emails depending on whichever has the largest hit/effort ratio. That they never seem to improve their emails means that either generating more leads is an incredibly effective way of generating hits, spam conversion rates are incredibly insensitive to content or spammers are not economically rational actors.

Of the three, the last seems the less absurd.

In a small scale marketing campaigns you can approximate that doubling the exposure will double the results, so talk about conversion ration etc. makes sense, but it will obviously not continue forever. Spam is just ridiculously cheap to send, what they optimize is total amount of replies, not amount of replies per spam. Every extra billion of spams gets you fewer marginal replies than the previous billion, but as long as it's worth more than tiny cost of sending it, it's worth sending that extra billion.

Also such tiny conversion ration don't really violate the facts of marketing. Spam has to first get through all the spam filters, which surely trash vast majority of it. Then you have to deal with people being trained to ignore or delete unsolicited emails. Then the best you can hope for are conversion rates you get from completely untargeted advertisement. And even when someone wants to get 12 inch cock, your rates will be lowered by their lack of trust in your ability to provide it, and their unwillingness to give credit card details to a shady website.

Add all these together and it's not certain that you can do much more than 125 million to 1, or that spammers are really all that bothered about having to send that many spams.