Because it's against the SQL standard (not that MySQL cares very much for that anyway). Just like SELECT * FROM table returns all the rows, DELETE FROM table deletes all the rows.
Let me rephrase: Why isn't the default mode of the mysql command line client to diverge from the SQL standard in the name of safety, with a command line option to enable "100% standard" (aka unsafe) mode?
I understand the SQL standard, and I understand the behavior. Why is the behavior of the software defaulting to "fuck up your life" in deference to some book on a shelf? (Especially considering you could revert to "fuck up your life" mode easily with a flag.)
