Why does String.split need pipe delimiter to be escaped?

Because the syntax for that parameter to split is a regular expression, where in the '|' has a special meaning of OR, and a '\|' means a literal '|' so the string "\\|" means the regular expression '\|' which means match exactly the character '|'.

String.split expects a regular expression argument. An unescaped | is parsed as a regex meaning "empty string or empty string," which isn't what you mean.