But sliders are impossible without effectively re-doing the entire gear system of the game itself, which in turn would require an engine overhaul. Which at that point, they'd just more likely to spend that time making the next FF MMO.

Characters in FF14 are effectively legos - there is no 'body' beneath your gear that your items warp around, the gear is your body. Like replacing a leg arm with a different lego arm, equipping a new piece of gear completely replaces that entire section of your character. This also means any visible skin on gear is built into the 3D model itself.

This means that butt sliders (and currently breast sliders) are useless, as they would only effect your smallclothes/emperor's gear and nothing else. Since every other piece of gear in the game overrides your slider choice with the model itself. This is why armor makes you effectively flat at 100%, and why Minfillia's gear gives you a great chest even at 0% on the slider: it's the actual model overriding it. You can even see this for the butt side of things with the 2B pants: They are intentionally sculpted to have a bigger butt, thus it only appears when you equip them.

For a butt slider to actually 'work' in FF14, they'd effectively have to create multiple copies of the same leg gear at different sculpts for each of the different races, then assign each once you hit a certain % on the slider. Which when you consider how long it takes them to do any retroactive gear update in FF14 (headpieces for Hroth/Viera, unlocking gender lock stuff, etc) is effectively impossible. This also ignores the other issue of having to make several new sculpts of body gear as well, since bigger butts would invariably clip through skirts/tops that are assuming the butt is at default size.

TL DR: Butt sliders are impossible with how their 3D model engine is set up. You'll have to turn to the dark forces of 3rd party if you want something like that.