I think you'd be surprised how much dedicated infrastructure and labor exists for A) spam, and B) anti-spam.

I used to manage email servers as part of my job, and >95% of traffic was spam. At the time, that seemed to be typical, judging from my discussions with others in the industry. Today I hear the number is closer to 50%, but I suspect that's because a lot of anti-spam measures are done further upstream (e.g. outgoing mail servers) so a lot of spam never makes it to its target server to begin with. And I guess spamming resources have moved somewhat to other protocols.

We spent thousands of dollars to get dedicated hardware to filter spam, along with a fat support contract for the spam-blocking software. Multiply that by the number of businesses that use email. (Of course, nowadays most businesses use cloud email from either Google or Microsoft, so it's much more efficient as far as anti-spam goes.)

At another job, I set it all up myself using open-source tools because we couldn't afford a fancy commercial solution. This reduced upfront cost but greatly increased the hours of labor I had to spend working on it.

Here's a study from 2012 estimating the cost of spam was about $20 billion per year in America alone: https://www.aeaweb.org/articles?id=10.1257/jep.26.3.87

We estimate that American firms and consumers experience costs of almost $20 billion annually due to spam. Our figure is more conservative than the $50 billion figure often cited by other authors, and we also note that the figure would be much higher if it were not for private investment in anti-spam technology by firms, which we detail further on. Based on the work of crafty computer scientists who have infiltrated and monitored spammers' activity, we estimate that spammers and spam-advertised merchants collect gross worldwide revenues on the order of $200 million per year. Thus, the "externality ratio" of external costs to internal benefits for spam is around 100:1.