that is.. why wordpress don´t use PDO if it is more secure, and prevent a lot of injections, and issues with security.
You don’t need PDO to write safe DB queries. PHP developers were able to do so before PDO was created ;)
There is nothing unsafe in using
WPDB class, as long as you do it correctly (just use prepare and correct ways of escaping).
There is one major flaw in your question: using PDO, of course, doesn’t make you secure - you still can write unsafe code ;) That’s why you can find SQL Injection vulnerabilities also in software that uses PDO (or even ORMs).
So I don’t see any reason for arguing why WP doesn’t use PDO. Much better (and more up to date) would be a question: “Why is there no ORM impelementation in WP” ;)