1) of course it is, sometimes even on purpose!
Some GMs use purely random loot generation, some hand-craft every creature's inventory and treasure chest, and a lot fall somewhere in between (per my experience).
With purely random loot generation, there's bound to be the occasional drop that's perfect for at least one of the characters (that first +1 sword is perfect for the sword-and-board fighter who's still rocking the non-masterwork sword they got at character creation).
Hand-crafted loot drops, unless the GM is explicitly trying to make the treasure not-immediately-useful, will similarly have immediately-useful stuff on occasion simply by chance. My experience also holds that GMs will intentionally sprinkle in items that specific characters will find useful, especially if the character feels underpowered compared to the rest of the party or just needs some help to properly fill their role (eg., a certain dwarven cleric I ran many years ago was trying to keep an oversized party alive; the GM dropped an artifact that boosted his healing ability to compensate for the ridiculously large party).
Additionally, pre-gen'd adventures are rife with dropping loot that will probably be useful later on: potions of break enchantment a bit before the party faces enchanters, an item that will let them rest peacefully in a dangerous dungeon, etc.. GMs writing their own adventures are entitled to do the same (and, in my experience again, are at least middling-ly likely to do so).
2) why should they?
More to the point, not all classes depend on treasure equally, nor do all classes depend on treasure equally across levels.
I ran a very long-lived campaign in which the wizard basically chose to not take any loot (aside from scrolls and a few cure potions, just in case) until around level 7, relying instead on their spellcasting and uncanny knack for finishing off enemies with a crossbow and letting the more martial characters get a bit ahead of the curve gear-wise.
The party was able to keep more of the more-expensive items and liquidate the less-expensive ones rather than having to liquidate expensive items in the interest of "equality", so the party as a whole had more wealth available to it; plus, it was a lot easier for the wizard to get dibs on stuff later on (including the "single item, such as a magic staff, [that] makes up most of the treasure") instead of trying to liquidate the fancy stuff so everybody had the same amount of wealth individually.
Which isn't to say that the GM shouldn't ever step in if the wealth distribution is wildly off and the "poor" character isn't on-board with the disparity, but it is this GM's opinion that they should typically do so with a light touch wherever possible - gently suggest that the party's overall survivability would increase without the disparity; it's all well and good that the fighter can one-shot a demon lord at level 10 because she's got all the party's wealth, but if the demon can command them first...
This is also where the "something the party can actually use" thing can come in; if the rogue's behind the curve and just can't find a trap to save their life, dropping an item that boosts their ability to do so might just be a good idea.
This GM does have one hard rule about loot, though: no stealing (thanks @Kommissar for reminding me). Characters are free to lend items, and it's possible that a borrowed item will be eaten by a rust monster, but once a character "owns" an item (ie., it's written on their character sheet), the only three ways it's removed without their explicit consent is (a) an NPC legitimately steals it (disarm, etc.), (b) another character takes it to save their life (pull a cure potion from the down'd character's bag to force down their throat), or (c) the PC is dead and won't be resurrected.