system
Given the following post, respond as if you're commenting on Reddit:
human
[Full Metal Alchemist] Why don't alchemists have a transmutation matrix tattooed into their bodies as a standard?

Watching for the first time and am only 3/4 of the way through season 2, so apologies if the show answers this later on:

The show presents Scar having the figures required for alchemy tattooed onto his arm as a novel concept. Then in the fight with Lust, Col. Mustang carving the circle into his hand is shown to the viewer as if it was an unconvential stroke of genius after Lust burns his gloves with the circles sewed in. 

It feels like having a circle readily available by having it part of your body would be incredibly useful, particularly for state alchemists who are sent into warzones. So why are these two the seemingly first ones to think of that?
gpt
There are others, but it’s not as common as you’d think. The ones that do are usually combat alchemists who use a specific circle a lot. Other than that, different transmutations require different circles. A tattoo would only be capable of one specific transmutation, like Mustang’s flame alchemy, so it’s usually only done by combat alchemists.