it is not about what requires more skill. it is about how much randomness items can add. (they span at random locations)
In competitive play, you can tolerate some randomness, but you usually want to reduce it, so that the more skilled player always wins, not the one that got lucky.
Any influence on results caused by luck or random happenstance can be ironed out by simply having more matches. If this sort of thing is truly random, than it isn't more likely to strike any given player. So, over a large number of matches, any statistical deviation caused by random chance is reduced to being insignificant.
In something like a round-robin tournament, where every player plays every other player at least once, one or two KOs caused by unpreventable circumstances isn't going to have any major effect on any player's overall standing. So, a random chance of something going wrong doesn't do anything to prevent skilled players from rising to the top.
Besides, the vast majority of item-related KOs are entirely preventable, and thus fall under the purview of player skill. If someone attacks a capsule sitting on the ground and it explodes and KOs them, than that is entirely their own fault. They just didn't have enough skill.
In melee it seemed to happen at least once every other match, maybe more. Gravity is too fast to really have time to react to it, and even if it wasn't, tons of attacks take place in the air anyhow.
I played hundred or thousands of matchs in Melee and Brawl with items on (often at high spawn rates), and I rarely die to random unpreventable accidents like that. Saying that they happen every other match is just confirmation bias. The few times that it does occur become over-exaggerated in peoples' mind's eye. There are also cases where people blame their own mistakes on the game. Blaming the game and its items is a way for people to side-step their own fault and lack of skill.
Having an element of random chance is perfectly fair. All sports and competitions have to deal with unpredictable elements to various degrees. As long as the random chance is unbiased, and affects all competitors equally, then no one has any right to claim that it hurts the competitiveness of the game. To go back to the Poker example, it is always possible for someone to draw a pair of Aces. That doesn't change the fact that skilled players will consistently beat unskilled players over enough games.
I think that any claim that items are "unfair" or "reduce skillful play" ring quite hollow. They don't. They merely change the nature of the game to something that some people may be uncomfortable with. It is fine to not like a certain variant of a game. But that doesn't make that variant "unfair" or not reward skillful play.