Can you please point to where WHO is saying people shouldn’t use homemade masks? I’ve never seen that claim, but if it’s true it indeed doesn’t make sense. I’ve only seen CDC dissuade people from using surgical and N95 masks.

Perhaps there’s concern that if CDC or others advise people to use homemade masks, people who actually do need N95 masks (e.g. severely immunocompromised) will think that works for them?

Edit: there’s also the moral hazard aspect to consider: if people think wearing homemade masks is enough to protect them, they’re probably a lot more likely to ignore quarantine or social distancing. If the masks aren’t 100% effective and are used wrong, that could be worse than just staying in and social distancing.