Oh, these are valid. You have to understand that there are a bunch of security consultant firms who are getting paid by these corporations to pick over the code to find every security vulnerability... the goal is to find something before it shows up 'in the wild', so the operating system can be patched.
But you need to remember a few things... . first, a lot of times, these security problems only exist on older versions of the software. The recent one going around, where malicious apps could get root access to your phone and install other apps, etc... they issue only affected Android phones that ran KitKat or older. Google addressed the issue in version 5.0 and above, but there are a TON of phones still on 4.4 or older because the OEM had long since abandoned the model and it hasn't received an update in years.
That's the heart of the matter... many of these vulnerabilities in Android are getting patched right away, but since OEMs are so excruciatingly slow to update their phones (if they update them at all), that leaves a huge number of them exposed. And the problem gets compounded by the fact that once the vulnerabilities are announced, hackers know that there are a millions of phones in use that will never see the fix.