Here's the first one. This one features some tables, since we can't just have one boring answer for all these pressing questions!
Why do you get experience points for killing things?
I already wrote up the reason for why gold = xp last time. But for this one we can give another reason too. Your characters have a supernatural "presence" associated with them which all things in the fantasy universe have. This is your Numen. By doing great deeds, gaining power, and generally challenging fate you essentially gain a sort of supernatural authority over the world- gently nudging it or warping it in ways you want. This power is subconscious; and subconsciously being who are defeated (not necessarily killed) can give up some of this spiritual authority to another being who bests them.
Why is old stuff so powerful anyway? - Roll 1d8
 The divine blood of the Gods existed in ancient people; it has been slowly degenerating since.
 Ancient, forgotten knowledge.
 They really aren't. The tales just got more and more exaggerated over time.
 Less pollution and people meant more wholesome magical energy crystals and stuff.
 Ancient people were aided by a race of heavenly aides/fantasy space aliens. They're gone now.
 People back then worked way harder, honor culture forced mastery. Nowadays, people are too jaded and cynical to care about shit like making the "best" magic sword.
 Reverse economics of scale. When your whole Kingdom relies on one exceptional warrior-hero, you'll craft him the best magic armor possible. Nowadays, we've got loads of Knights.
 The world has a certain amount of ambient power- trapped and crystalized in objects, spells, and beings. As time went on, it gets thinner and thinner, like reverse ozone.
Why do Elder Gods want sacrifices if they're so inscrutable? - Roll 1d8
 To prove your dedication; only way to win an audience with one.
 Cultists do them to steel themselves to speak to and experience the eldritch. Hardens the mind.
 The Elders don't understand people that well; assume the reason such bloody sacrifices aren't done often is because they have some kind of value.
 Elder Gods are simply cruel and evil, not just ambivalent.
 The mental state of a tortured and dying person is uniquely fascinating to psychic entities
 These beings are like bottomless pits, no amount of life can fill them, but they want more.
 To attract their attention. Intense emotions draw them in- technically any emotion works if strong enough, but anguish is easier to make on demand then happiness so...
 It's pure superstition by the cultists. The first time they got their rituals to work was with a sacrifice, so now they do it every time.
Why can Magic Users only prepare spells in certain slots? Why can't they fill greater spell slots with weaker spells?
I actually like this one being a mystery because it makes the magic system seem more mysterious. If you needed and explanation; it's because the different energies and parts of the brain the magic user has to prepare spells in are different and non compatible.
Why are mutants/nonhuman races tolerated in fantasy human society when in real life humans barely tolerate each other if that?
I've never liked this line of thinking. In a fantasy world, people would naturally be more lenient because it's more common. In the real world, there are no elves and mutations are exceedingly rare (and pretty much always debilitating). In a fantasy world, these races exist and move around sometimes, plus mutations and weird body shifted people, or people changed into talking animals or whatever is a thing. If it happened at least once, then it's happened before and people will be more used to it.
Why aren't the Elves overpopulated and basically take up the whole world?
In many fantasy worlds, Elves as a race can live forever or for thousands of years, and elves are often among the oldest races as well. So why isn't everyone an elf? Some people point to long lifespans and gestation times, but with elves being fertile for most of their lives means that logically they would be among the most populous races in a fantasy world.
The solution to this one is really simple; elves have a quirk of their biology. Couples can only have a child if they have true love for each other.
Why do Elves think they are better then everybody?
Because they are better then everybody.
Why is the world stuck in Medieval Stasis? (Optional 1d6 table)
This is one I've never really understood, as plenty of worlds have a fictional fantasy bronze age before them, and a fantasy stone age/time of gods and shit before that. So the world isn't actually in stasis, we're just currently at the medievalish time period for this world.
Of course, sometimes these fantasy worlds have the whole world stuck at the same roughly-medieval or early renaissance tech level for like thousands upon thousands of years, which strains believably. The old standby for me is that the world simply isn't like the real world, the laws of physics are different; the materials to make gunpowder just don't combust if put together, for example. But if you need some more reasons- here's a little table for you.
 The world is sustained through exceptional individuals. Since they die, no advancement.
 The fantasy elements (gods, magic, level-ups, etc.) mean that technology isn't very needed.
 Wizards and other magic-users have created a sort of secret cabal to prevent them from being made obsolete by technology.
 The Gods reset mortals when they get too powerful. The reason? So they can stay in control and not be overthrown by their creations.
 The Gods reset mortals when they get too powerful. The reason? To prevent mortals from destroying the world with paradox technology and/or demon invasions through portals.
 The Gods reset mortals when they get too powerful. The reason? They know how shitty society will become after the industrial revolution.
Why don't monsters actually use all the powerful items in the dungeons they live in?
I already did this one.