1. Asymptomatic spreading
People can have disease for up to 14 days without any symptoms. In this time they are spreading it to other people. So asking people with symptoms to stay at home does not work.
2. No safe number to prevent transmission
Even 2 people congregating there is risk of spread. There is NO safe number of congregation (even 5 or 10) so that is why any congregation – not just mosques is discouraged.
3. Fomites are super-spreaders
Mosque carpets, prayer caps, wudhu taps and so on can have virus living on them for 1 day even for a few days. If cleaning is not done adequately – it will lead to spread.
4. Risk is to people at home not just in mosque
A healthy person in mosque who picks up virus, can take it back to their unhealthy relative and cause their death without intention.
5. We cannot afford mosques to be associated with thousands of deaths
Please see impact of Tableegh on Indian Muslims. If public associate spread of virus with Mosques then it creates conflict between public & scholars.