...in what way did you compare that?
Marksmen have an issue with time that they need to solve, both in the long term and in the short term. With the exception of the AD Casters who have Mage-like qualities, Mages scale much faster and Burst through much more damage in a far shorter amount of time than a Marksman can compare. You can say that given an adequate amount of time, a Marksman will outperform a Mage, but given an infinite period of time and a Marksman outperforms every other possible option in the game; such an argument doesn't make sense to hold over a Mage's head.
Second of all, Fighters, by their very name, are often designed and balanced around their ability to Fight. Throw most any Fighter vs most anything else, Mage, Marksman, whatever, into a cage and make them slug it out, and the Fighter will almost always win. Now, consider for a moment why we don't use Fighters instead of Marksman in bot lane. Now, consider that I just compared Mages to Marksman. You should be able to figure out why a direct comparison between Mages and Fighters is kinda a bad example.
Third of all, Assassins are designed around the principle of countering anyone squishy in general. Comparing once again to Marksmen, if you're a glass cannon and an Assassin so much as looks as you, you're probably screwed regardless of whether you output your damage through abilities or auto attacks. Furthermore, the fact that we have Bulkier or Lockdown Mages designed in turn to counter Assassins like Lissandra or Malzahar or Cho'Gath if we want to be technical, to say that "Assassins counter Mages" is far too wide of a statement that doesn't actually hold true in plenty of scenarios.
