A medical mask used correctly needs to be replaced every 4 hours, binned without touching the surface. Not many will use them correctly without training, so current advice is simply not to bother. It would lead to a false sense of security.
Advice given recently on this topic is that 'medical' face masks are only intended to avoid contaminating another person, when you may have something that may be harmful to them. They're not designed to protect an individual from external infections, and may be ineffectual for this.
Also, the effect of a false sense of security is counterproductive. Of course, a face mask may be effective at keeping others weill away from you (social distancing) in public places, but that will depend on how many others are also wearing them.