The game just doesn't work like that. Many "side" missions directly tie into the main missions by being necessary to unlock Legendary Beasts or Cultists. These side quests are also often presented in exactly the same way as the main quest, consisting of a lot of dialogue and storytelling. If you just want to concentrate on doing the "main" missions, you would end up at a huge roadblock at some point where two out of the three "main" mission plot lines would be unobtainable since they directly tie in with you exploring the world and finding clues by doing quests and, well, exploring the world.
In Assassin's Creed Odyssey, main missions and side missions are mostly the same. They get the same production values, the same interesting storylines and the same gameplay mechanics. And since exploring the world and finding the cultists by interacting with people asking for help is part of the main quest line, the distinction between those and the "side" content doesn't really make sense in that way here. The side quests aren't side quests to a large degree, they are just as much main quests as, well, the "main" quests.