Google does not "allow" security issues. Its engineers are still human and humans are imperfect creatures, which means they sometimes make mistakes, not notice completely everything, or do not think about all of the cases (they try to be as perfect as possible, but you just cannot be perfect, especially when you have a code base that includes millions of lines of code).
That means that -
- Security issues creep into the code base (but a lot of them are fixed, promptly when they are discovered).
- Security issues may have always been in the code base. Remember that Google Chrome uses a slew of third party code (it was audited for security, but the imperfection argument still applies), as well as Blink, which is a fork of WebKit, which is a fork of KHTML, which includes code from the nineties, which also might have undiscovered security issues (but a lot of them are fixed, promptly when they are discovered).
So, the hard truth is that Google Chrome (and any other browser and most of the software and hardware in the world) has security issues. Due to being open source, fixed security issues are pretty much well known (like in Firefox and Safari and other open source software), so you definitely want to get Chrome (and any other browser!) updates, unless you want to risk being hacked.