You know, if you were approaching this as "Wow, casters are so much weaker than they were in 3.5!" I don't think anyone would even bat an eye. You are right, casters are a lot weaker than they were in 3.5
But, at the end of the day, you aren't getting the problem. Sure, maybe the fighter isn't doing 1d8+8, maybe they are doing 2d12+3d6+12. But, at the end of the day, they are only capable of dealing damage one attack roll at a time. If a fighter comes to a door, then their plan is to open the door and attack whatever is on the other side. And if they lose initiative, then they will attack on their turn. And if they win initiative, then they will attack on their turn. And if the enemy outnumbers the fighter... they will attack on their turn.
I'm actually reminded of a situation a friend of mine dealt with. He was transported to a demiplane to fight by a powerful being. He was a high level cleric. He was fighting zombies. An actual endless horde of zombies. We stopped keeping track of how many zombies he killed. He unleashed all his destroy undeads, all his castings of blade barrier, then started in on spirit guardians.
A fighter wouldn't have had any option other than attacking, one zombie at a time. My friend's cleric never even got touched by a single zombie, while killing hundreds of them. A fighter in the same situation? Likely would have been killed, because every turn he'd have been attacked by 9 zombies, without fail.
Casters have more options. They don't have to just attack AC. They don't have to just deal damage. They can do those things, and do them incredibly effectively, but that is not the ONLY thing they can do. It is the only thing pure martials can do. They have no other tools.