Why won't my generator start
Clogged carburetor, 90% of the time, especially with modern gasoline/petrol.

You need fuel, air and spark for a gasoline engine to run. You have checked the air filter; you have changed the spark plug, but probably not actually verified spark (it's common to get a harmless but irritating shock while testing that, but you can avoid that with clever use of a bungee cord to hold the removed from engine, connected to wire spark plug's base against a metal part of the engine, rather than using your hand) and you have checked fuel flow from the tank - but if the fuel is not getting through the carburetor due to small ports and passages being clogged, it won't get into the air stream.

Another possibility is that the generator uses a manual choke, which most younger folks are not all that familiar with and may forget to operate as a result.

Rarer with modern controls/engines is a flooded (too much fuel) engine, where a few cranks with the throttle closed will help to clear out the excess fuel.