Any world that has a history of thousands upon thousands of years doesn't need to explain itself. Literally anything could happen in a timespan like that; all you need to do is establish for the characters that they are exploring something ancient and abandoned and forgotten and they know that anything can and will go.
I agree. Sometimes people seem to forget that they are playing in ancient fantasy worlds populated by cultures alien to our own (and not just the cultures of the non-humans, either) and fail to acknowledge that they might behave in a way that fall outside the realm of our expectation.
This is not to say that I think the imaginations of people who prefer 'realistic' games are somehow limited. It's much easier to relate to and play characters in a world where the motivations and cultures of those characters are not so different to your own -and for some people, it's more fun, too. I do think, though, that to claim something is not realistic because it does not conform to your view of what is plausible on earth is at the very least doing yourself a disservice - you're closing yourself off from some really fantastic ideas and worlds.
The sad thing is that some fantastical ideas are quite 'realistic' - if you just take a bit of time to explore the possibilities, and often things in the 'real world' seem so implausible that you might not believe in them - had they not existed your whole life. Things in our world gain instant credibility because they are, well, real - but consider how outlandish these things might sound if your GM had just come up with them for his world
- 5 thousand years ago, in one of the earliest known societies, thousands upon thousands of slaves spent over a decade creating truly a colossal quadrilateral tomb for their the kings. It went so well they decided to keep on doing it. The largest was made of 1.3 million, 2.5 ton, limestone blocks and was almost 500 foot tall.
- On the other side of the world, similar structures were built (by a society with no contact with the first) and used as the site of mass human sacrificial ceremonies. At one ceremony, thought to span 4 days, over 80,000 people were said to be murdered.
- In a distant land, a bizarre mammal-like water creature has warm blood, fur and expresses milk, but it also lays eggs, has a bill and it's webbed feet have poisonous spurs.
- Under the feet of everyone who lives on the world is super-heated metal and rock. Every so often this spews forth from the tops of mountains and flows down the hills in liquid form killing everything in it's path. People still decide to live alongside these mountains
- Millions of people spend part of their weekly income tickets in a regular competition based on the random selection of a series of numbered table tennis balls. This is despite the odds of an entry claiming the biggest prize being 1 in several million.
- There are so few grains of sand on the beaches of this world that they are outnumbered by the number of Stars in the universe around it. The GM explains that this is partly due to the relatively large size of the universe - 9.3 × 10^10 "light years"(a term for distance, apparently). It seems a bit further than "a walk down to the chemist", but you would have to think that there aren't many beaches on the world.
Look, I could go on. So many things that are real seem so unreal. Some are mind bogglingly incomprehensible - and that includes the insane and bizarre things people do to each other (or themselves) in the name of their country, their king, their god, and so on. I avoided mentioning these on the whole because I don't want to derail the thread, but seriously, people do the oddest thngs. Especially when they are super rich, or when they are grouped together.
On top of that, other people then come along who spend their whole lives exploring and investigating these things, dedicating themselves to some subatomic particle, a genus of of insect that only lives in a particular patch of desert in the middle of nowhere, or the varnish on a brand of violin. This is quite odd too, when you think about it, but real as well! These people do because they dedicated to figuring out what makes the world works. What makes people tick. Where we come from. They have inquiring minds.
So, if you come across something a bit unrealistic - in your GM's game, or in a post on ENworld, may I suggest you take a leaf out of their book and take just minute trying to work it out for yourself. Don't rely on the GM or other posters to do the work for you. Come up with a reason yourself. And given that we are discussing a game, try to focus not on proving something to be wrong, but instead look a reason why it might be right.