Why are face masks open on both sides?

Most people are liars and talk out of both sides of their mouth.

I think that the theory is that people still need to breathe, and that indirect (sideways) airflow is probably less dangerous that the straight-out airflow.

You have to be careful when buying a face ask to make sure it filters enough & that it covers your face sufficiently. The one I purchased goes from ear to ear as well as from covering my nose to going under my chin. It has a pocket so I can slide in a filter. The mask itself can be washed & the filter can be changed as needed.

Surgical masks are to protect other people from your saliva infecting them not to protect the wearer. If you need to protect the wearer then different mask is required (N95 in the US / FFP3 in euro)