The Marauders found every secret passage out and into Hogwarts and every shortcut, hidden passage, and hidden room in Hogwarts, how did they miss the Room of Requirement?
It's possible they did.
It's possible they never had need for the Room of Requirement. The one time that I can think of that they might have needed the Room of Requirement is when Lupin needed a spot to hide his Werewolf problem, but Dumbledore figured that out for him.
"But then Dumbledore became headmaster, and he was sympathetic. He said that as long as we took certain precautions, there was no reason I shouldn't come to school..." Lupin sighed, and looked directly at Harry. "I told you, months ago, that the Whomping Willow was planted the year I came to Hogwarts. The truth is it was planted because I came to Hogwarts. This house" - Lupin looked miserably around the room - "the tunnel that leads to it - they were built for my use. Once a month, I was smuggled out of the castle, into this place, to transform. The tree was placed at the tunnel mouth to stop anyone coming across me while i was dangerous."
- Harry Potter and the Prisoner of Azkaban, Chapter 18 (Moony, Wormtail, Padfoot, And Prongs)
Another possible explanation is that they did know about it but couldn't put it on the map because it is unplottable, and the room has rules.
The beauty of the Room of Requirement was that it adjusted to nuances. Even the Marauder’s Map couldn’t tell you who was inside it.
- Everything you need to know about the Room of Requirement - Pottermore
To me this quote means that even if the Marauders did know about it it was protected magically against being put on their map.
It would not be especially troublesome if the Marauders had never found the Room of Requirement. If you don't know about it, the only way to find it is pretty much sheer dumb luck. To quote the original description of how to access it, from Chapter Eighteen of Order of the Phoenix:
"Dobby said to walk past this bit of wall three times, concentrating hard on what we need."
Apparently, then, there are three conditions to accessing the Room of Requirement:
- You must be in front of the wall.
- You must concentrate on what you need.
- You must walk past it three times.
The odds of these three things converging together by accident would seem to be pretty small. While it does seem that a few people (e.g. Dumbledore and Filch) may in fact have stumbled upon it by accident, there doesn't seem to be any reason to think that this would be likely to happen to any given person – even someone who spends a lot of time exploring the castle. In fact, Voldemort assumed that he was the only one who had ever discovered it. From Chapter Twenty-Seven of Deathly Hallows:
As for the school: He alone knew where in Hogwarts he had stowed the Horcrux, because he alone had plumbed the deepest secrets of that place...
Additionally, it is perhaps arguable that even if all three conditions are met, the room will not necessarily show itself. Dobby says:
Sometimes it is there, and sometimes it is not, but when it appears, it is always equipped for the seeker's needs.
Depending on how you understand the first four words of this quote, it might mean that even when you do properly seek it, it will only be there sometimes.
Either way, it is entirely reasonable, even expected, that even though they "found every secret passage out and into Hogwarts and every shortcut, hidden passage, and hidden room in Hogwarts" they would not have found the Room of Requirement.