I don't think any tabletop roleplaying system mechanics can be "realistic". Skill levels, attack and defense rolls, damage, experience, modifiers - none of them have a damn thing to do with realism. Mechanics are abstractions - even the ones that most people consider complicated - and in no way approach realistically simulating anything. As such realism is a complete waste of time as a design goal. If you're trying to create a ranged combat system to realistically simulate firearms, just fucking stop now. The same goes for sword fighting, or vehicle combat, or anything else. You're just going to wind up plastering "Most realistic rpg ever" on the cover and get mocked. Maybe go play a video game instead if you want that kind of realism.
Others just go for silly
One of the defenses of realistic rules is that the GM often doesn't have experience with a particular circumstance - whether it be rock climbing, or driving a tank, or firing a gun, or hacking a computer. Therefore, the realistic detailed rules provide the framework for the GM to be able to adjudicate those situations. The problem with this approach is the designers often don't really know either (cf Vampire Undeath's being advertised as realistic and thoroughly researched, with a rule that a jammed assault rifle needs five minutes and an armorer to clear). This extends to many things like how difficult tasks should be (how many times have you seen a system with task thresholds that just seem out of whack), how often people succeed at tasks (ditto for people failing at tasks, but this is usually a dice issue), the number and scope of various modifiers, and how people learn. The designer might have pages of equations backing up why a particular modifier works the way it does and even that will be a white room affair that doesn't take into account countless variables.
That isn't to say it's not a good exercise to incorporate whatever level of realism into actually designing the mechanics. It might even help balance out some rules and figure out where the boundaries of believability are. But once that tweaking and honing is done, the consistency and playability need to be the ultimate goal - not whether or not the game accurately represents the unladen weight of a swallow.