I have read, without a lot of evidence to be fair, that port capacity is a small part of it. Overseas container shipping companies had, pre-pandemic, been competing so heavily that they were often running a loss and hoping to make it up on volume. They used the pandemic to sort of consolidate and lower capacity by decommissioning ships and so dramatically increase per-container price. Not sure if there's a lot of clear evidence for it, but it's one explanation I read.