First: Browsing around the website, all books I could reach from the homepage or by searching for titles I could think of are available in multiple digital formats.
Second: PG is older than PDF, EPUB, MOBI, and even HTML. It isn't too surprising to me that there are works there that are text only.
I remember reading plain text books from PG at work ~20 years ago. The network was monitored so I had to bring them in on a 3.5" floppy. Having probably read thousands of pages that way, it's a bit odd to see plain text described as "unreadable".
Second: PG is older than PDF, EPUB, MOBI, and even HTML. It isn't too surprising to me that there are works there that are text only.