In short, it's because spammers use Judo throws against the mail providers who, in their exhaustion overreact. Generally during a flood of spam what providers will do is whack an entire /24 of IPs, taking the "Kill 'em all and let god sort them out" strategy. It is for similar reasons many servers flat-out block IPs originating from other countries.

Anyhow, this has lead to plenty of headache for services which market themselves on deliverability. Mailchimp, sendgrid and Amazon SES all have to keep a hold of far more IPs than they need at any given time to keep themselves squeaky clean. They also have to rate-limit and aggressively ban anyone sending what looks like spam either by AI analysis or bayesian filtering. Spammers on the other hand aren't dummies and they have vast resources at their command. It's straightforward to brute-force reverse engineer which markov chains actually get thru, as mail servers normally tell the sender why they failed to deliver a message.

At a certain scale this becomes a real problem. After a spammer has crafted a message they know will sail past the filters, they then can hook up to the "reputable" mailers as relays in parallel and shoot out a huge wave of un-interceptable spam before anyone can do anything about it. Everyone in the value chain gets mad, and massive overreactions happen thanks to this harming the bottom line.

The most important of these is to return 250 OK to messages which are known to be bad, and then silently deleting them. This leaves the spammer none the wiser. It's essentially a "hellban" where they will simply waste resources drilling a dry hole. Crafting un-interceptable messages is essentially impossible under this regime, and the spammers have to go back to their old tricks of smearing wide swaths of previously clean IPs with their garbage.

On the other hand, legitimate servers in these IP blocks get run over. Even servers which have been on the same IP for years get overwhelmed reputationally as they would have to send out far more volume of email to build enough reputation to overcome the insane volume of spam coming out of sibling IPs. Worse yet, there is no sign of anything wrong and no recourse whatsoever. You simply find out your email has NOT been delivered a few weeks later after its cost you and your business untold quantities of money.

As such the only real way to protect yourself is buy a huge IP block, and basically don't use any but one of them for mail. It's a "you must be this tall to ride" mechanism, like much of the rest of the internet has become. Either that or you have to sidle up to a firm which has a big IP block (and ruthlessly protects it) via forwarding with things like postsrsd & postforward.

In short the only cure for spam that has worked is steadily increasing the marginal cost of sending a message. Many feel anxious about this, as they realize anything they have to say is essentially going to be held captive to large firms. The weak are crushed into their usual condition of "nothing to say, and no way to say it".

As always, prosperity is a necessary condition of your freedom of action. This being a somewhat uncomfortable truth is probably why the reality of the Email situation is seldom discussed.