Developing worlds, populations with millions living in poverty. Billions of people without the money to buy things two weeks in advance. Millions of food producers without the facility to produce and store 2 weeks worth of things for people. Now consider enforcement, healthcare, transportation, logistics, communication, borders, active wars and civil wars going on right now, etc. Let alone the normal international political cooperation needed.
It's actually totally impossible for it to happen simultaneously at a global level. I'd say it's probably also impossible for the majority of countries in the world (i.e. not rich) to have an effective lockdown at all at any time.
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.