In the 80's, design in RPGs was wide open and we barely had language to talk about the issues of design. There was a lot of good design and a lot of bad design, but one very common complaint about D&D is that the design was bad because it wasn't 'realistic', and there was a general sense in much of the community that many if not all of the problems tables encountered in a game was do to a lack of 'realism'. Various systems of Traveller and GURPS might also be worth looking at, and a trip into the world of GULLIVER (a modification of GURPS) would also be worthwhile. This led to a fetishization of realism as a goal of game design, which can be seen in extremely complex games of the period - HERO, and Rolemaster might be a very good examples, though the pain points in that complexity come up at different points. Since the goal of RPGs started out as basically 'Simulation of the World', the attributes of a system which were considered very admirable in a system were that it would be universal (able to simulate everything) and realistic (able to produce a simulation that produced intuitive or 'correct' results).
@
Celebrim sums it up pretty well here, with the key idea being,
why the need for complexity? What does the complexity actually positively accomplish either at or away from the game table?
Ostensibly the goal early on for creating more complex games was realism. The thought was that in order to more accurately "simulate" real-world processes and phenomena, you had to create rules systems that operated at a highly granular level.
The problem with doing this is that implementing that complexity
at the table---making skill checks, running details combat scenarios---became so cumbersome that many players ended up rejecting the resulting play experience.
If "realism" really was the end-all, be-all to a quality RPG experience, then Rolemaster, GURPS, HERO, Runequest, etc., would have long ago gotten a much larger footprint into the TRPG "cultural identity," but they haven't.
What little experience I have with these types of systems is with GURPS, but I was always struck by the irony of the attitude many of the players had when I was in that particular group. They had this attitude that GURPS was the "One True Way," that anyone who didn't immediately recognize and embrace the awesomeness that was GURPS was essentially a moron. "It's so much more realistic," "You can literally do anything with it," "I don't get why ANYONE would still play D&D when GURPS is around; D&D is inferior in every way."
But after a few brief turns of popularity, 33 years after its release in 1986, GURPS is barely a niche player at this point. D&D, Pathfinder, Fate, Savage Worlds, Fantasy Flight Star Wars / Genesys are all objectively more popular and widely played based on sales numbers and play statistics on things like Roll20 and Fantasy Grounds. I'd even argue that the combined OSR and Powered by the Apocalypse games are vastly more popular.
And why is that? Because for other than a microscopic fraction of the TRPG player base, the complexity of GURPS does not ultimately serve the purposes of play for participants.
So the question again is, why are you adding complexity? It's already been shown through decades of real-world experience that "complexity for the sake of realism" is a dead-end goal. So why else would you add complexity?
Are you wanting to just give players more options to muck about with away from the table? Because it's no denying that this was a huge draw for D&D 3.x / Pathfinder. But that again only plays into the needs of a small subset of gamers.
Are you, as @
Celebrim noted, adding complexity because it makes a good marketing strategy? Because this was clearly the case with GURPS as well, where they literally have a supplement for every conceivable genre and historical period. And that's fine, but again, SOMEONE has to balance all that complexity against the costs for putting it to use in play.
(*Edit* As [MENTION=4937]Celebrim[/MENTION] mentions too, historically there was a strident section of RPG players at the time who firmly believed that adherence to realism as the ultimate goal would create dramatically better play experiences across the board. The reasoning being, most disagreements at the game table between players and GMs were around "how realistic" stuff should be, and that if GMs could just grasp "realism" better, that games would automatically improve in play. And we continue to see this impulse manifest itself, even now.)