You're comparing end game source required spell to a level 1 sorc cast rain and then attacking with a earth or lighting wand and thinking, "hmmmm there's some disparity here", but also ignoring the ease of access to mass aoe.
I was using Arrow Storm as an example because it's available around the same time as Meteor Shower, which is the most powerful elemental spell. Or at least Meteor Shower and Hail Storm are anyway. Prior to that, Rangers have Barrage and Ricochet, melee have Battering Ram, Battle Stomp and Whirlwind (although it's still almost always better to single target enemies down if they are at full health until very late game).
Physical has the advantage of having double damage bonuses from a weapon skill and warfare, the primary physical skills also confer their own bonuses.but it can't do damage to people for just walking. It can't block los, heal armour efficiently, or just dump on people.
Mobility tax is mostly an issue with Warrior builds, not nearly as much with Scoundrels. Not only do Scoundrel builds naturally walk a bit more per AP, Backlash gets them to their first enemy pretty early and is probably the most effecient ability in the game since it's an 8m teleport by default (which can be improved with Far Man Out) that also happens to backstab for a single AP. The Pawn is also somewhat mandatory and does a lot for melee, where ranged get more use out of Executioner.
Rangers on the other hand have to Tactical Retreat at worst, and that's basically a free skill going into the next round. Scoundrel also has access to its own long range teleport earlier than Warfare (Warriors can detour into Scoundrel too) and eventually they'll both have Phoenix Dive and Cloak and Dagger.
There's more tactics involved with magic for sure, but there's also more tactical possibilities. It's not as clear cut as just use physical. Your life is undeniably easier with magic as some people are weak to it and you don't need max rank source powers to do it.
There's more
effort involved. Not tactics. There's a difference. I'm currently playing a four elemental team and it's a comparative slog, and it also requires more buffing and healing in general (and I'm wearing shields on all of them too).
And why are you excluding summoners? They're part of the game they help a caster be a flexible contributer to any party.
Because Summoning is an outlier, and scales very oddly. I know how strong they can be, because I've played one all the way through now twice. In a split group they are ridiculous for versatility once you get elemental infusions (and it's also sad that the Incarnate does more damage with its own spells than what a player can at a similar skill level for a very long time, and for less Source too). In an all physical group they are kind of garbage aside from having a beefy distraction that can once per battle knock down an enemy or maybe finish off a half dead Mage with a ranged attack. In an all elemental group they become
extremely powerful, but again the scaling is really odd.
They start out as all physical, unless you combine the books you need to get the basic Water, Fire, Poison and Electric buffs or summon them inside of an element (which isn't realiable). If you don't grab those books early, you basically play a physical damage dealer with an elemental character.
Even if you do get the infusions you need, they need around 5-7 points dumped before they can really contribute much of anything, and you need to double buff them on top of any potential infusion. You basically spend your entire first turn summoning and buffing your avatar that may or may not actually do 2-4 APs worth of actual damage during its first round.
The longer the fights last, the more efficient the Incarnate becomes (but then that says a lot about your killing effeciency too...). They finally become great once you hit Summoning 10 and beyond, and the 1-2 spells that they can cast up until maybe two-thirds of the way through Act 2 hit harder than what the player can, but of course they won't be casting as many spells overall. From there, player characters (and I'm talking those that are focused on dealing elemental damage too
from the start, not some Summoning 10 then focusing on a single school type of caster either) start closing in and surpassing the elemental damage that a buffed Incarnate can do with the same spells. Finally.
The same issue happens with the physical version too, except quite a bit earlier. The physical version's damage varies wildly depending on how up to date your own physical damage dealers' gear is. If it's within 1-2 levels of the character level, the Incarnate's damage falls off once the player is around level 13 or so. Sometimes a bit earlier depending on vendor and drop luck. At end game the Incarnate is probably going to be the beefiest player on the field for physical and magical armor as well as health, but it's also most likely going to be the lowest damage too thanks to weapon scaling.
Of course there's also even more specialized summons like the Bone Widow (which hits like a truck, but is 100% CC bait), the Slug that lets you laser beam two or three times in a single encounter and the Artillery Plant, which has some insane first round burst against non undead, but they won't get as much use due to basically being a one tricky pony while the Incarnate has a utility belt. Plus, two of them always cost source, while the Incarnate doesn't have to for every summons.
Anyway, a group of four Summoners will be able to steamroll through the game almost as easy (in some encounters easier) as an all physical group. That says nothing about the personal damage or CC the caster is doing however as it's mostly the Summons doing all of the work. Eventually you'll also be able to max out a single elemental school to go along with your Summoning, but by that point the game is almost over anyway. Summoner focused builds make better buffers and healers than anything else for most of the game.
