I was reading about phylogenies and found many literatures said that outgroups contain the ancestrial trait, but why is that? Aren't they also experiencing the same amount of evolutionary time and (presumably) changing their characters as well?
Yes, sister taxa are, by definition, of equal age. So they evolve independently after the two groups diverge. You are asking about character polarity, and you are exactly right. That is why multiple outgroups are better for inferring which state is ancestral. If an outgroup is state A, and a second more distant outgroup (an out-outgroup) is also state A, and the ingroup has state B, then you can be fairly confident that state A is ancestral. If you look at dozens of characters this way, you will find that any of the three taxa in any combination may have the ancestral state. Imagine the distant outgroup (the out-outgroup) is state A, and both the close outgroup and the ingroup are state B. Then again, you could not know which state is ancestral, and would need a further outgroup. These are inferences, not measurements.