@Ana1234 - The problem is, now more than ever, hospitals need to be aware of infections, particularly of those so-called super-bugs. From what I've read, in order to sterilize surgical equipment, they will put them into a special heating unit that will reach enormous temperatures to kill off anything that might be on there, without relying on antiseptics.
Venturi masks aren't made of strong enough stuff to withstand the kind of heat needed to really kill off all the bugs and become hospital-grade sterilized. And even if they were, I suspect the process of sterilization would be more expensive than just making a new mask.
I think in an ideal world, what they should do is recycle the old masks by melting them down and making something new. I'm almost positive most hospitals don't do this, however.