Oh wait, mine don't sour. To get those bacteria to work to make sours you have to inoculate the beer. They just don't infect every beer. The conditions for them to propagate just aren't there in beer.
Please read carefully to understand what I'm saying.
Most of the time we can prevent contaminants because
most problematic organisms won't grow in beer and there's a
low probably that they will make their way into your primary fermenter
if you are careful.
The notion that beer
cannot become contaminated is simply false.
These contaminating organisms are ubiquitous, floating around in the air. They don't need to be intentionally inoculated, all they need is a chance to land in the sweet wort.
A little bad luck is all it takes, especially if someone's cleaning, sanitation, or brewing practices aren't ideal.
Plenty of homebrewers use risky techniques like no boil, no chill, lots of air exposure (lengthy aeration), underpitching (long lag time), poor temperature control (encourages bacteria), poor cleaning methods or even utter lack of cleaning/sanitizing steps, etc.
Telling these brewers that beer can't be contaminated is really not encouraging good sanitation practices.
Just because you and I have been fortunate enough to avoid contamination doesn't mean others are so lucky, especially those that take more risks.