[mkl]

Here's why I think animated films are done like that.

The best/only way to get most of that computer-generated imagery is by huge amounts of manual labour: designing, animating, simulating, sometimes motion-capturing. It's painstaking detail work involving many people.

The best way to get the voices is with a small amount of manual labour: voice acting.

If you put as much manual effort as the imagery into controlling the nuances of a TTS engine, you might get acceptable results, but it's far easier and cheaper to use voice actors. In fact, the easiest way to tell a TTS engine exactly what you want would probably be to voice act and have it mimic you. This might be worth trying to do if remapping vocal anatomy (e.g. woman voicing man or vice versa, or monster, etc.), but for most purposes it's easier to hire appropriate voice actors and/or manipulate the vocal recording audio than to use it to drive a resynthesis by simulation.

[SuperGent]

Also, it is a way to get A-list celebs involved and cash in on their popularity.

[pcroh]

Maybe someday we will see (or hear) Siri's voice star in one of those Disney flicks.

[minxomat]

Weird example since Siri is based on one real woman's voice. Maybe one of the WaveNet "personalities" might be a better example.

[have_faith]

Or the Japanese voice idols?

[Filligree]

Vocaloids?

Those are deliberately made to sound unnatural. Not to say it changes anything, and they've already shown up once or twice in anime.

(Though the only example I can name off the bat is Black Rock Shooter, and that doesn't include the voice. It's complicated. Mato is complicated, too.)