A mask that's capable of keeping a virus out is a a) much more expensive than a typical surgical mask, b) easy to wear improperly, negating its usefulness, and c) only good for a limited time before needing to be thrown out.
The best way to protect yourself from the illness is to stay away from other infected people, not wear some home made protective gear that you're probably not going to replace often enough or handle and dispose of properly after you take it off.
All the doctors and nurses having to wear masks will be so relieved to hear this.
But seriously the advice of not wearing N95 masks when you are healthy is not because they are ineffective, but because they are trying to discourage hoarding. Even current CDC recommendations for healthcare workers suggest N95 masks for only patient intake and direct care, with all others wearing surgical masks to help protect the supply of N95s.
The difference is, healthcare workers are trained in the use of N95 masks, undergo annual fit testing, etc.
Additionally - this [2] new search shows that if 80% of uninfected persons wore a mask for a few days, the pandemic would be over.