Log files are intended to aid debugging of issues. Typically, hard drive space is much cheaper than engineering time. Log files use text because there are many tools for working with text (such as
tail -f). Even HTTP uses plain-text (see also why don't we send binary around instead of text on httpwhy don't we send binary around instead of text on http).
Additionally, it's cheaper to develop a plain-text logging system and verify that it works, easier to debug if it goes wrong, and easier to recover any useful information in case the system fails and corrupts part of the log.