More broadly, this is a consequence of English being a LTR language. The eye scans from left to right when reading these languages, so one naturally wants to pl

More broadly, this is a consequence of English being a LTR language. The eye scans from left to right when reading these languages, so one naturally wants to place important visual symbols (bullets, controls, etc.) firstmost where they'll offer maximum contextual impact for the proceeding information. The fixed-width nature of these elements is more of an afterthought that emerges because neat grids make information easier to parse and are generally considered to be visually pleasing. In RTL languages, the movement of the eye is mirrored, so the natural position of these elements also gets mirrored to the right-hand side. Of course, the fixed-width nature does not change because the instinct to craft neat grids exists regardless of directionality. Recently, however, we have begun to see certain touch-based interfaces adopt right-aligned controls specifically, even in LTR configurations. This evolved to compensate for the fact that most people are right-handed and generally cannot reach across the width of a modern touchscreen one-handed. It's effectively a concession to accessibility and is best implemented as a toggleable feature to avoid disadvantaging left-handed users.