Why do orthoesters react with Grignard reagents, but acetals and ketals don't?
Grignard reactions are performed in ether or THF.
There you have only one -OR group on a carbon and, since it is the solvent, it is not reacting with it.
This shows that not always a -OR group is substituted in a Grignard reaction.

When you have three -OR's on one carbon, the formal positive charge ($\delta+$) on it is higher than when there are only two -OR's. 
Therefore, the reaction proceeds on orthoesters and not on acetals or ketals.