For some it's laziness, for others it's just not knowing what to say. Sometimes I struggle with the latter too like I simply don't know what to say. Especially if it's ongoing fics and I kinda want to comment on every chapter but then sometimes it's just hard. Sometimes you're in a hurry (I read a fic once before and in the break of my sister's theater performance - not while they were acting but...) and you simply don't have time to write a comment. And then I guess some readers just don't consider their words of appreciation worth anything because "the author knows they're good enough" ^_^