The roof of your bedroom could fall off on you every night, however, it hasn't so far and you just assume that it won't because not assuming so would make your life horrible by say, making you sleep in the outdoor, which may be even more dangerous. Life is full of compromise, of course the next version of Google Chrome could wipe out your hard drive and post all your photos to 4chan without your consent, but based on a few evidences (Google being a reasonable company whose interest is not to mess with you, past positive experience with chrome etc) we choose to compromise and accept autoupdate because it's convenient and we consider the amount of evidence enough to trust it.