It's not to be the perfect security, just make spamming harder.
Spammers will have to scrap lots and lots of pages to create a database of email addresses. It's cheaper to get the text, search for a pattern like
email@example.com and add to the database than to search for
name at server.com, or
name at server dot com, or
name at server . com, and other variations, parse all, validate all and add to the database.