In my humble opinion, there are two major reasons:
1) Larger Scope - As someone already mentioned, RPGs are very large and complex games, consisting of a lot more parts than other genres. In a shooter, the devs can just focus all their time into the combat system and level design, but in an RPG, they have to do combat, world design, dialogue, character development, various non-combat mechanics (for RPGs that have that). They also have a lot more content. Whereas a shooter can be under 10 hours in length (or under 30 in the good old days), a decent RPG is always expected to be at least 40 hours, or over a 100 for large open world ones. So the dev's time and money, which are limited resources, are spent on a lot more stuff in RPGs, leaving a lot less for any given system.
2) The fascination with stats - In a shooter or an action-adventure game, the devs can just focus on making gameplay fun. For a combat system, they can figure out what shooting model feels good and is fun for the player, and just put it in, for example. But if it's an RPG, the devs feel like they have to tie the model in to the character's stats. Whenever something becomes more stat based, it inevitably becomes less fun. For a very simple illustration of this, consider a pure stat based combat system vs a pure skill-based one. In the former, you just compare the stats of 2 opponents and everything is auto-resolved. In the latter, the player has to actively control everything and demonstrate mastery of the system. This is a simplified comparison, but you get the general idea. Also, stat introduction into system makes things much harder to balance, as stats change over time. This is why shooter and action-adventure combat is almost always more fun than RPG combat. The sad thing is, this fascination with stats is absolutely unnecessary, as there are other ways to enforce roles in RPGs. For instance, allow the character to acquire abilities necessary for the role over time.