Even with a global lockdown, there will always be a reservoir of virus somewhere.

Some immunocompromised people can carry the virus for weeks.

Also, what about households where one person catches COVID the day before lockdown, incubates for a few days (asymptomatically), then transmits it to another person in the household, who also incubates for a few days...

At the end of the two week lockdowns, some people will still be contagious.