The time spent making them really shows in their quality and heart.
They also take a really long time.
Most films take years to complete, when you include all of the time from when the person has the first idea to when the film is finally released.
In Pixar's case, a large fraction of the time is still dominated by getting the story right. They're known for continually rewriting the storyboard for years until they're happy with it.
Even once the storyboard is done and the voice acting and animation is ready to begin, Pixar doesn't just use off-the-shelf computer software to do their animation. They employ hundreds of programmers who specialize in writing software to generate new 3-D effects.
Each new film pushes the boundaries of what's possible in computer-generated 3-D graphics - more realistic and nuanced textures, more detail, more believable movement, plus new creative special effects that would never be possible in real life. They spent hundreds of man-years just making the fur for the movie Monsters, Inc., as one example.
That means that there's lots and lots of Man hours that go with into making the films. As well as computing hours. As the technology progresses so does the complexity of the films.