People receiving email don't feel it should be their job to stop spam
There is a popular observation when people get spam, especially spam from places that specialize in sending email such as Sendgrid, and that is that these places are not infrequently pretty good about handling spam issues. Sometimes they have 'complain that this is spam' links right in the email headers (not Sendgrid, though), or similar things. But thinking that this is sufficient is a mistake, perhaps not in the short term but definitely in the long term (in my opinion).
What places that are merely 'good at responding to complaints' are really doing is making it the job of people receiving email to deal with spam. Unsurprisingly, not infrequently people are unhappy with this, either quietly or loudly. As a general rule, people do not want it to be their job to deal with spam; they want it to be your job (where 'you' is some combination of the people operating their mail system and the people responsible for sending email to them). That in practice it currently is their job, requiring a bunch of labour and a bunch of annoying workarounds and hacks, is something that people are not exactly pleased about. They consider this a flaw in the system, and it is.
(Spam in general is a fundamental hard problem, but spam from major mail sending providers is merely because they aren't interested in trying hard enough to stop it.)
Of course, people receiving email do not necessarily understand the situation in the way I've put it here. But we do know that they are unhappy to get spam (there are very few people who are happy to get it); even if they are 'happy' to be able to do something to make it go away, they would be happier to not have to deal with spam at all. The more work they have to do and the less certain that work is in making the spam go away, the more unhappy they are, until you reach the stage of someone like me.
PS: A great many things are 'making it the job of the receiver to deal with spam', including the very basic one of 'I think I will use GMail and live with its drawbacks because they have really good spam filtering'. Certainly not taking genuinely effective steps to stop spam senders and then waiting for the complaints to roll in is one of them; the mail sending place has effectively outsourced a significant part of its spam detection to the people receiving its email. If said people did nothing, the mail sending place would not know it was being used to send spam.
PPS: To cut off one possible reply in advance, a claim of 'if no one complains, clearly it wasn't spam' is demonstrably and obviously false and anyone advancing it as a defense is either morally vacuous or doing so in a cynical manipulation to deflect criticism. That marketing and advertising organizations love to make claims like this is one reason why a great many people hate them.