Casters to replace the role that an non caster can do, needs to spend an spell slot. And an wizard needs to have knowledge to prepare. And Wish, is a tier 9 spell. Takes an long time to get Wish. Also, Wish is very DM dependent. You can't for eg "i wish that i an a god"(you can, but the DM can make you become an immortal slime demigod being burned eternally on plane of fire, or simple say that the spell failed because <<insert any reason>>). And items can give this "powers" to martial classes. Also, depending your heritage, you can get wings.
No, casters don´t need to prepare a spell slot necessarily
Items also give powers to caster classes, In fact most non-martial classes cannot use them unless they spend ponts to do it in the build: scrolls, wands and rods.
Same as always, you need to change your build and level-up to do the same as a lvl 1 wizard with a bunch of wands and scrolls.
Except that WANDS aren't for free and can't cast any spell. I don't know much about 5e, but on Pathfinder, they require an use magic device check(, that is a CHA based check(
http://www.d20pfsrd.com/skills/use-magic-device/ ) and an wand of stoneskin for eg costs 21k gold. enough to buy 4 +2 belt of giant strength (
http://www.d20pfsrd.com/magic-items/wondrous-items/a-b/belt-of-giant-strength)
This not mentioning that wands can only "charge" spells up to tier 4 (
https://www.d20pfsrd.com/magic-items/wands/ )
So, you can have an wand of fireballs, but no wand of stop time, wand of wish, wand of create greater undead or etc. Say that an lv 1 wiz would have access to this wands/scrolls makes no sense. Is like say that an half orc barbarian with 22 STR raging with an adamantine fire greataxe can outDPS an wizard with 6 CON/6 STR at mid levels inside a Antimagic Field... And on Divine Divinity, i an playing as an mage and i an pretty versatile. Can animate 3 skeletons with skill level = 3, and looks like even the skill that allow you to become invisible isn't on CD. You just spend a lot of mana / second(din't unlocked this skill yet)
The point of magic in fantasy is the same of technology IRL. Allow humans to do things that they could't otherwise. Arcanum was an interesting game, because both are opposite. Technology is the application of nature laws. Magic is the negation. So, if you focus on tech, you can create an steampunk army, make steampunk grenade launchers, just like magicians can trow fireballs and conjure servants.
