In Power Rangers: Megaforce, the world is under siege by an Alien Armada. The Aliens launch multiple assaults on Earth and the Power Rangers were all that stood between them and conquest every time. So how do five people who live in the same city and seemingly never travel, defend the entire Earth? I understand that Power Rangers is a kids show and this may have never been addressed, but is there and in universe answer?
Megaforce may do the best job explaining this. Gosei is a very powerful wizard with advanced technology; he is able to teleport the rangers to multiple locations fairly easily and often. Early transportation frameworks seen in previous shows, for example Mighty Morphing, were mainly Home City -> Base and Base -> Home City (exceptions exist).
Later on in the series in Super Megaforce, they get flight vehicles like the Super Mega Skyship and the Legendary Megazord that travel amazingly fast. When searching for Turtle Island to get the Red Lion Wildzord, they circumnavigate the planet many times in a short period of time.
Throughout most of the series though the home city is the primary focus. The villains often explain or allude to the reason why they attack the heroes' home city: the Megaforce Rangers are the largest threat to the conquest of Earth.
To clarify that last point, why the enemy focuses on the rangers, it is important to differentiate between Megaforce (Season 1) and Super Megaforce (Season 2).
In Megaforce, the primary antagonists have one ship: the Warstar. And for reasons I won't go into, generally there is only one Monster of the Week. It is irrelevant whether the villains attack the home city or not (sometimes they do, sometimes they don't) since the rangers can teleport.
In Super Megaforce, the primary antagonist is the Armada. At the beginning of the season the villains do send forth a global assault. The shiny new Legendary Megazord repels the attack, severely damaging the advance fleet. From that point on, the enemies know that it is throwing away resources to do a mass attack. The Legendary Megazord will just glass the fleet again if they do. From that point on, it is wise to send elite troops (commanders or higher) to eliminate the power rangers so that the fleet may swoop in and conquer.
From memory most Power Rangers series have something that the villain needs before their big invasion or they live in a location villain needs or sometimes just a personal grudge against the Power Rangers is all that keeps them coming back.