A lightning bolt that makes all players kill-able at around 50%.
A green turtle shell that acts as a fast projectile that kills around 85%
Two hammers that kill at low percentages, and one allows you to glide.
A fan that easily racks up a shit ton of damage and is nearly impossible to DI out of
A soccer ball that acts as an extremely fast and deadly projectile that can kill at low percents
A heart that manages to heal 100%, which is the majority of kill ranges.
A giant bomb that takes up almost half of the stage and racks up good damage
Home run bat becomes an incredibly fast and hard hitting projectile
Shit, almost every item becomes a fast, hard hitting projectile that can be easily aimed and gimps the majority of recoveries
Bomb ombs become throwable KOs
Pokemon who's attack take up half the screen and deal massive amounts of damage
Spicy Curry is a damage racking machine
The bumper has RIDICULOUS knockback and could kill at 50% (without DI, but still)
Cracker Launcher was a fast shooting, powerful projectile machine you can easily spam
Ray gun is pretty much a low risk instant kill given how easily it kills by getting someone off stage and spamming the A button
Crates and things pop out of no where and can even spawn in the middle of an attack and explode. There's a video I posted before that's a good example of this
The hammer head is a projectile of doom
Bananas cause tripping, and while I don't consider them too OP they're so useful a top tier character's play style is centered around them

All of the items that have drawbacks (such as sqeaky) happen at such a small percentage it's almost a non factor. I'm pretty sure that they happen at such a low percent, they occur less often than GW's OHKO move. That move has a much higher chance of happening, and it's seldom used by GW players (which are pretty few given his tier ranking) because it's so unreliable and the OHKO has a higher percentage of happening than the powerful weapons' downsides.

I might also add that the not only do these items turn the tide of a game, they do it with much less skill than it does to turn the tide without items. Getting away from your opponent and pressing A on an item before he does takes considerably less skill than a zero to death combo someone pulled in desperation

All of these items are arguably OP, especially considered to the move sets of the characters. If items were the norm, the gameplay would no doubt become centralized on the items because they're OP compared to the move sets. It's not about "adaptability," it's that a lot of the items are arguably OP and the majority of competitive scenes try to eliminate randomness