You... haven't played many side-scrollers and RPGM games, have you? For games that are primarily keyboard operated, it's standard to have movement on the arrow keys, and a number of abilities on the left side of the keyboard, primarily because in those games, especially side-scrollers and platformers, precisions is especially needed with movement, and there's a bias for right-handed people rather than left-handed.
The whole W,A,S,D with mouse operation scheme popped in much later in gaming, with FPS games and certain RPGs, as well as some side-scrollers that use the mouse for aiming.
On top of that, this is an RPGM game. Mouse support for RPGM games is actually relatively recent in the grand scheme of things. It didn't exist at all before... MV I think? And even then, it was pretty crappy and some devs, especially for H-games, didn't use it at all because it was finnicky or because they were familiar with working without it from previous RPGM versions. Hell, WolfRPG, the other RPG-style-maker thing is even worse, and it's very popular over in JP. From what I've seen, putting mouse-support in that is hell.
So, nah. While modern gaming trends in the past 20 years or so use the WASD scheme with mouse, that's only for a specific subset of games and not every game out there follows it. Modern platformers, from what I know, still use the arrow-keys primarily unless they're the type that works with mouse (and those are more of a side-scroller shooter than proper platformers).
Of course, modern games allow rebinding keys to allow all sorts of set-ups for comfort... but that too can be tricky to do in RPGM, so I'm not surprised that a random H-game doesn't have that sort of feature.