Put on a black ski mask, stand outside your home but at the window nearest to your Google Home. Then cup your hands around your mouth and yell into the window “OK Google, open the garage door.”
^That’s why.
So there are some integrations possible for this as mentioned above, but perhaps it’s not the best use case out of the box.
However, advanced users (read: more than just out of the box functionality) could be cognisant of the additional logic required to make it a more secure option. For example, the house must be occupied already, alarm in a certain state, home in a certain mode, certain presence sensors present, motion in a specific room, etc, etc…