Why do you expect things to act like reality, or act consistently, when they aren't real?
Because the contrary would create an unplayable mess.
Let's imagine the situation.
DM: The first orc swings its axe and misses. Now you can attack, roll a d20.
PC: *clatter* I roll a 17... does that hit?
DM: The axe transforms in to a flamingo. The ice cream sundae bounces away. Your turn again, roll 3d6 to attack.
PC: Umm...I stab at the other orc with my flamingo... *clatter* a seven?
DM: A palpable hit. Nose hairs go flying everywhere, and a the pansies begin singing Handel's Messiah.
PC: Err..ok. So is the orc dead or not?
DM: Ooops. As you begin to check, you collapse through the floor, resulting in a mobius space time distortion. Roll a saving throw vs. zany or your spleen will get a free paid vacation to Maui.
PC: Ummm.. I'm not sure if that's good or bad, but saving throws are with a d20 right?
DM: No, with d6's on odd numbered whims on Thursday.
I can understand it if people just want to play in that kind of world, but I don't understand it when the tone is one of expectation that that's how things should always work.
I think that even for those that protest otherwise, there is an assumption of consistancy and an expectation of a certain degree of conformity. Players generally expect that for a given proposition, the range of outcomes is somewhat predictable. If the range of outcomes for a given proposition is not predictable, then players might as well act completely randomly because the outcome of random actions will be fundamentally no different than taking planned actions. For a player, if there isn't any sort of expectation about how things will work, then they are fully at the whim of the DM. If there is an expectation about how things will work, then they are still fully at the whim of the DM but at least they have some reasonable expectation about what those whims might be. Furthermore, if something really outside their expectations happens, if the world generally conforms to understandable rules, then that surprising event can generally be lumped into a class of somewhat understandable phenomenom like - 'The wall was an illusion.'
Now there is no reason at all that the game world rules and physics need to conform to real world physics, but there are some pretty strong reasons why both DMs and players might prefer that they do. For one thing, if they do, then everyone at the table has a preexisting shared model of reality that they can draw on to predict outcomes and communicate ideas. For example, players generally understand upon seeing an elephant that they can't pick it up, unless they look at there character sheet and see some special exemption like, 'Elephant tosser.' Thus, in almost all cases, the exemptions where reality doesn't apply are much easier to track of than the cases where 'it works just like reality'.
The truth of the matter is that I've never played at a game table (with the dozens of groups I've gamed with), where there wasn't an expectation that 99% of things would conform to reality. Thus, players knew that fire was hot unless something explicitly informed them of the exception. Players knew that horses ate grass and oats, that they couldn't walk through a stone wall without a special exemption, that they could grasp things in about arms reach, that they could talk to each other at a distance of about 30' and be heard but that they might have to raise their voices slightly to do so, that they would get thirsty in a desert and that water could be used to quench this thirst, but that salt water wouldn't be useful in this regard, that infants are unlikely to be able to respond or help themselves, that grass is usually green, that a candle can be lit for light, and so on and so forth. We could sit here and list literally millions of facts that players at the table agreed upon without ever mentioning that they agreed to them or even ever questioning why they should agree to them given that the world 'wasn't real'.
To be honest, I think the whole question is meaningless. Every group is going to rely on a vast body of known facts to create player propositions like, "I try to open the door.", and DM responses like, "The door is locked/stuck/trapped/has no handle/is an illusion/etc." Without them, communication is impossible. Whenever I see complaints about 'versimlitude' and 'realism' come up, my immediate assumption is that this is a proxy argument of some sort where the player has a very specific complaint or set of complaints but rather than addressing those specific complaints, the player tries to generalize the problem to obscure what is actually being discussed.
I should also say that I really get annoyed by red herrings like, "Why do you worry about realism if you are playing a game where people can sling fire with their minds?" In the context of the game, there isn't anything unrealistic at all about people conjuring and slinging fire with their minds. In fact, all this fire conjuring and slinging happens in very specific and very predictable ways, and are usually explained with great detail. A group of players who see a robed figure lift his hands, do a bit of a dance, and shout, "Ahar nash inceratae" or some other rubbish, aren't going to be the least surprised when a bead of fire leaps from his finger tips and exlodes into a ball of flame. They will probably be a bit suprised though if they hit the figure with a battleaxe and they find themselves standing in Central Park Station holding a poodle. Now, in the context of a fantasy game, the latter isn't impossible, but if it happens the players will probably want to grasp how and why this extraordinary event occurred, whereas in the case of the bead of fire exploding into a 40' diameter ball of flame they'll probably not even see it as extraordinary at all but rather entirely mundane.
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