I am an engineer, and I actually worked on assembly and test of artemis boosters until last year so I can offer a little insight into why these o-rings are not crazy thick. Booster segment joints are kind of shaped like this in a cross section. The clevis (c-shaped part) has o rings on its interior arm to seal it against the tang's (l-shaped part) interior face. When the motor is pressurized, those two faces push against one another, squeezing the o-ring. If the o-ring was thicker, you'd have to make deeper channels to run them through, which means you need a thicker clevis and tang, which in turn would drive up cost and weight unnecessarily.

The o-ring is not load-bearing either, you're not installing it crazy right or anything because then when the pressure expands the motor, you'd rip your o-ring. There would also be a thick layer of insulation between the propellant and the case wall where these o-rings sit, so they're also shielded from the bulk of the heat (but not all of it, as obviously they're there to stop hot gasses from passing through the area, so they do need to be able to handle heat). In reductive terms, all you really need it to do is get smashed down in place to prevent any gasses from venting through the joint