Because Hayao Miyazaki wanted it like this:
The most striking thing about “The Wind Rises,” though, are the images
rather than the politics. It’s easy to fall under the spell of the
sheer beauty of the film, with its soaring vistas and an array of
sound effects — trains, airplane engines and the like — done
by mouths, not machines. “Originally Miyazaki wanted to have all the
sound effects voiced, and he wanted to do them himself,” said Wexler.
“They said ‘No, we need you to draw the film.’ - THEWRAP
This doesn't serve any real value to the film's deep analysis and was even criticized by some.