Why do anime films tend to use film actors to voice the anime instead of professional seiyuu?
There are two aspects we can discuss on this topic.

Some directors like Miyazaki prefer to hire film actors.
He said anime seiyuu is too optimized for anime and does not sound natural.
Seiyuu is good when the target audience is anime fans,
but if they want to get a broader audience, actors are the best choice.

Another aspect is getting attention by using famous actors.
Producer Toshio Suzuki is known to always use actors instead of seiyuu.
For example, he used Takuya Kimura in Howl's Moving Castle.
Takuya's acting is not good, but it gets more audience because of his name.