Mass Effect gives player character Commander Shepard free rein to explore the Milky Way, including different systems and planets. Unfortunately, not all of the planets the player can find in the galaxy are worth exploring. There’s good reason for them to exist, but even so, they’ll find that most of the planets they come across are empty and unremarkable.
Most of the planets in the Mass Effect version of the Milky Way are uninhabitable and don’t have much happening on their surfaces. For every system the player es through in the Normandy, they’ll find one or two planets that are described as simply having a “calcium-heavy crust” or being “an unremarkable world.”
Some of the worlds in Mass Effect will have Easter eggs or lore-building information in their flavor text. For example, the flavor text for a planet called Eingana reveals the existence of two pre-Prothean races called the Inusannon and the Thoi’han (both of these races were later wiped out by Mass Effect's Reapers).
Not Every World In Mass Effect Can Be Explored
As for why players can’t land on or explore every planet in the Milky Way in Mass Effect, it’s economic game design mixed with realism: The worlds of our solar system wouldn’t all be worth exploring to a galactic adventurer, so why would remote systems be any different? And if the player could land on every planet, that’d cost a massive amount of time and resources that the developers of the time (or, for that matter, developers of today) simply did not have. But there is still value in these unremarkable, uninhabitable planets out there in the Milky Way, both to the player and to the characters in-universe.
The ordinary planetary bodies, even when they don’t have life-sustaining atmospheres, can be mined for resources. In Mass Effect 2, they can be scanned and probed for minerals such as platinum, iridium, or the precious element zero that creates the titular Mass Effect; all of which can be used to upgrade Shepard's ship, the Normandy SR-2.
Gas giants are usually the most boring Mass Effect planets on any given system, though they do fill an important in-universe purpose: Ships can eject their drive cores - essentially jettisoning the electrical charge built up from flying - in the magnetic field of gas giants. Helium-3, an important component of the nuclear fusion thrusters that propel a ship, can also be skimmed from gas giants. The Normandy never needs to do this (not in Mass Effect's story, anyway), so you won’t see it in-game.
The galaxy in Mass Effect is filled with beautiful and interesting worlds. Players may not be able to land on or visit all of them, but the developers have done their best to make sure that even the boring planets have something to offer, even if it's just a rich vein of platinum or an obscure piece of lore.