[Nursie]

Healthcare workers wear masks of a known quality, which they have fit tested, and they know how long to use them and when to dispose of them. They wear them as part of a full protective strategy along with other context-appropriate equipment and have training to know how to conduct themselves in the situation too.

You can't just claim the same benefits for an old piece of t-shirt stretched across your face. An old piece of t-shirt which may give you a false sense of security and lead you to take risks and get closer to other people, and which may make you touch your face more, and which you may not clean or replace etc etc...