because GMing is always similar.
My experience is counter to this.
GMing a game with a unified mechanic (GURPS, MegaTraveller, Traveller: The New Era, WWG Storyteller System, Savage Worlds) is different from running a game that's a bunch of discrete special case rules and 3 core resolution modes (AD&D, Classic Traveller), even in the same settings.
Classic Traveller (CT) has several different mechanics in rules:
- 2d6+custom mods for custom TN+ by specific skill;
- 2d6 ±1 for Att + Skill level +Wpn'sArmorMod + Wpn'sRangeMod ≥ 8 (Combat);
- Nd6 < attribute+skill (strongly alluded to in Bk 0, and used a couple times in adventures, but per Marc Miller, his original intent);
- 2d6 ≤ rating modified by attributes (some skills, encounter checks),
- 2d6 + mods for high attributes ≥ listed TN (Character prior experience).
This is usually simplified by GM's to a variation of #2: 2d6 ±1 for att +Skill for 6+/8+/10+ or 5+/8+/11+. The latter is typically those who used Striker/AHL in place of Bk1 combat. And most keep #5 in that context.
Some go more flexible... but almost none use all 5 modes in play.
Meanwhile, MegaTraveller (MT) has one core resolution: 2d6 + AssetA + AssetB for N+ to succeed, (3d6-(AssetA+AssetB))× BaseTimeIncrement for time taken. Assets may be any of ①Attribute/5 ⤵ ② Skill ③ Ship's Computer Rating ④ other ratings in the 1-8 range.
There are 7 difficulties - 5 explicit, two implicit. So Auto, 3+, 7+, 11+, 15+, 19+, fail. So, running it, there's a lot more consistency of capability, but a strong tendency to mechanicalize many things not using Mechanics in CT, despite the same setting. It also tends to make GM's go more skill rolls and fewer "You're skilled, you succeed."
Due to the way tasks work in Combat, MT is a MUCH deadlier game than CT, too. A .22LR is able to kill all but the strongest in MT, as it can do up to 8 dice of damage to attributes, and a peak roll of 48 exceeds the max physicals of a human of 3×15; CT, it only does 2d to attributes. The basic rifle in CD is 3d, while in MT, it can be 1, 2, 3, 6, or 12 dice. Both games, injury reduces attributes, and since attributes are generated on 2d6... and only STR, DEX, and END normally take damage...,
Then, comparing that to T20:Traveller's Handbook, using the d20 system... was another feel entirely. Neither like CT nor MT, nor D&D... A different range of potential damages, a different range of skill effects, different attribute scaling, the system of Feats.... I run all three in the same setting, the game feels different to me as a GM, and to my players, because the nature of the character ratings and how they change play are different. We playtested T20...
Now, for a GM less prone to reach for mechanics, this may not matter outside combat...
The "sometimes we even roll the dice" GM isn't playing the ruleset 90% of the time; they're telling a story informed by the character sheets. That's fun for some; it's not what I'm there for. I want the mechanics, the game, to matter. And so the result of different mechanics is quickly felt when I run... both by the players and by me.
We have a problem with not having enough GMs, and part of this problem is that games are designed by GMs and not by typical players, because GMing is always similar. I would never want to GM, because the way most games are designed for GMs to be god and I really dont like this kind of elitism and dont want myself to be associated with it in any form.
You really should read more varied games. GM authority is quite different in most of the PBTA/AWE games, and in games like Cosmic Patrol, where GMing rotates. quite a few post-2000 games explore different GMing limits and final authority (several put the final rules authority to the table as a whole by vote).