It's simple. Having to hide behind cover most of the time (not moving) because of lethal hitscanners -> boring. Shooting enemies while dodging various kinds of projectiles -> not boring.
You're always moving in both instances, it's just a question of the magnitude of the movement - micro camera movements and subtle re-positioning, vs running around the arena at lightning speed. We're just arguing a difference of taste here, so it's probably not worth going in circles for that. And, for the
third time, I recommend trying out Half-Life Mmod to really see the difficulty that renders this a not quite so black-or-white point. You get more freedom and control when you enable projectile aiming, but lose out on a good chunk of the lethality of the gunfights. It just makes the game easier and less measured, and as a response to dedicated map design in Doom, my next point...
Good level designers are aware of the strengths and weaknesses of different enemies, and know how to use them to their advantage and create interesting encounters. Players are expected to abuse the simple AI by hoarding enemies, staying in a certain spot for a while to pull enemies away from walls and create space so you can move behind them. Well designed encounters won't allow you to simply circlestrafe. You have to earn it. Add some Revenants, and your movement completely changes because you have to avoid homing projectiles on top of dodging non-homing projectiles. Put some Archviles in strategic places for soft area denial. Force the player to move behind cover for a few seconds to avoid the AV's attack without getting hit by all the projectiles. You can't get those situations when all the enemies are highly lethal hitscanners.
The big health pool of Cyberdemons is usually balanced by either giving the player strong weapons (BFG) or use them for infighting (another lost art in FPS games) which weakens them. There is also a trend in modern maps to use a new type of Cyberdemon that has only 1/4 of his normal health.
I just played the first couple of levels of Sunlust on ultra violence, and while it is a little more interesting than the vanilla Doom 2 experience, it's by no means the revelation you seem to want it to be. My strategy usually involved sprinting full speed into a room, then sprinting out since most of them triggered traps and hidden enemies in walls. Then it was just a matter of retreating to a previous room and doing the typical left-to-right shooting by a wall, which works flawlessly until your ammo runs dry. I said
nothing of circle strafing. Sometimes you get fucked over by a trap and are killed, but then the surprise is ruined since the map always plays out the same, so you just trigger it and immediately book it so that you can funnel the idiot mobs into a straight line (assuming they don't get stuck on a wall, back exposed for an easy kill) and, rinse-and-repeat, find the wall of your choice and, left to right, left to right, left to fucking right. I would run out of ammo sometimes because I hadn't memorized the enemy layout, so then it was just using berserk in the same way by a wall, running past the idiot waves to scavenge for ammo, or just getting overhwhelmed, dying, and trying again, only to lessen the challenge since, once more, the levels are completely static and nothing changes. Archviles do mix things up a little bit, but their fire is easy to work around and anticipate, so you just bait them into using it and deal with them in much the same way as you do everything else. You aren't adapting, you're memorizing, and that's just way less interesting. It's like autistic speed runners, it's the antithesis of the human experience to just suck all novelty out of one rigid, unchanging challenge.
Edit: And dear God, the music in this mod is the most grating shit I've heard in a good while