Nobody started being a longshoreman in the last year and a half. Last summer, basically nobody said, "hmm, I'm young and strong and I'm okay with doing serious physical labor, also my family's worked on and around ships for generations - I guess I'll be like my dad/uncle/older sister/etc. and get a job as a longshoreman." (Even if people wanted the job: Nobody was hiring last summer. Businesses weren't open, weren't shipping anything other than necessities.)
It's always been a job with heavy attrition. People get injured, get sick, get disabled, and quit. And new young-strong-hardworking people sign up for the next opening.
Nobody's replacing them now.
And in the past, there's never been a heavy push to get people to sign up. There are no "become a LONGSHOREMAN!!!" posters in the career center. It's widely know as "what you do if you have brawn and not much else going for you." It pays just enough to keep people from spending their off-work hours looking for something else, not enough to actually convince people to sign up if they have an array of other options.
...Everyone is now looking into other options.
And the laborers in the shipping industry - longshoremen, truckers, warehouse workers, delivery people - have always been treated as expendables, as fungible human resources soon to be replaced.
If you want to join the schadenfreude popcorn club, watch the news media go into increasing panic mode as they try so very very hard to talk about "supply chain failures" without talking about the people who made it function for so long. Watch politicians and business moguls wring their hands and mumble about "but what can we do???" while dancing around the topic of, "um... maybe provide a living wage and health care?" without acknowledging it exists.
Of course, none of them will get as far as "maybe provide BONUSES for people who agree to do the work you all believe desperately needs to get done." No, they'll talk about providing "incentive" to business owners, not to the workers.