True, but both they and other military personnel could be deployed to help/free other hospital staff (ambulance/paramedics) and alleviate the burden. However, in the UK, a paralyzing fear seems to persist that deploying the military in a civilian setting, in this case the NHS, is tantamount to admitting that the system has broken.
The military have been and do appear ready to help, so it's simply a case of making the request and giving the order.
https://www.forces.net/news/coronavirus-how-military-helping