Casters get more powerful spells, but generally never get more accurate or higher saves. (I really liked when the level of the spell influenced the save; it gave a reason to "upcast" even when you got nothing else. It served my literary need for "I pump a lot more power into it, to try to land the spell".) Excepting a couple very specific items, the casters can't go pick up a +2 wand or +1 holy symbol to get a bonus to hit or damage.
Martials, however, can pick up a +1 weapon and suddenly are "5 levels" more accurate in the Bounded Accuracy system of proficiency bonus. (+1 damage, whoop whoop, meh). As others have pointed out, the 5e designers didn't want to include that "issue" in their monster design, nor go back down the 4e / Pathfinder slope of "level 10 PC can't be hit by a kobold; level 10 PC can't hit the CR 20 demon".
As a DM, this has incentivized me (again, as other people have said across 10 pages of this thread so far) to create "magic" weapons and armor that are weird/interesting/useful without tacking on a mechanical accuracy bonus. The party's first magic weapon was an argentium (mithril-like material) warhammer with inlaid jade chips (jade is "anti-aberration" IMC for story reasons). The weapon inflicted +1 damage undead and aberrations, and was treated "as a magic weapon" for overcoming resistances; if you hit an aberration, a portion of the jade inlay would detonate for +1d6 damage [on a "6", all the jade is gone, and you need to get more for 50gp]. No bonus to hit.
The second magic weapon was "Pinion", a truly +1 magical cold-iron javelin of goblin-make. But being a rod of solid (and poorly-made) rod of iron... it was naturally -1 to hit! So again, no inherent accuracy bonus. But it does +2d6 damage against Fey and Elementals! (And has a minor range benefit -- if it was thrown at Short Range last round, this round you can throw it at Long Range without disadvantage.)
The third magic weapon was a "gambler's blade" dagger. When you attune to it - each morning! - you pick whether it is a +1, +2, or +3 magic weapon! The catch is that your Death Saves for the day suffer the same penalty as the bonus you picked... (The dagger also gives double bonus in necrotic damage to the target on a crit... and highlights you both with faerie fire for a round.) To date, 5 levels later, no one has attuned to the weapon.
Several party members wear a special-material Light Armor that is functionally "+1 studded leather", picked up in a trade deal around 7th level. The warlock has "enchanted paldrons" of lightning attraction he wears, that provide +1 AC but make him vulnerable to lightning. The fighter/rogue has a shield that glows on command and makes him "1 step" (usually 17%, i.e. 1-in-6) more likely to be targeted than his allies (i.e. "minor taunt"), and his breastplate causes 2 damage to any undead or fey that touch him. He usually wields an argentium shortsword (+1 damage to undead and aberrations) that is now enchanted to do an extra die of thunder damage when he uses booming blade - but sometimes he uses the dagger of "mageborn slaying" that senses sorcerers and creatures with innate magical powers, and is +2 to hit and does +1d6 acid damage to them. He also carries a captured antimagic spear of petrified wood from a hobgoblin warlord that casts dispel magic on a hit, for special cases.
I could go on. The point is, over a 9-level campaign, the martials (and the casters, in some cases!) have gotten "interesting" gear early and often, without necessarily getting "+X to hit" benefits. I feel like this is the sweet-spot in 5e. Give the players "sugar-free candy", and they are happy getting cool stuff without "breaking" 5e bounded accuracy. Martials especially like the variety, particularly because it doesn't require them to become "second-rate casters".
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I've been DMing since 1983.
