This is mostly because authors can't resist the urge to also do something "interesting" in addition to "just" syntax highlighting. imenu support for example. (imenu is "tags without a tags table" for one file.)

If you look at cperl-mode, for example, syntax highlighting and indentation support is only a small part of its line count. The rest is things like auto-documentation of core features, including 100s of lines of documentation strings for Perl core functions.