Because losing in a skill based game due to a random number generator is totally fair.
Patching out critical hits is a bit much, but fighting games shouldn't really have randomness in the mechanics unless it's implemented like Teddie in P4A or something.
Games like DOTA are all about random number generators. It does add a skill requirement to the game because you're required to have knowledge of how, why, and when the RNGs work and how to use them to your favor. It's "fair" because you're expected to know how to work around RNGs that put you at a disadvantage and to be prepared to react appropriately when things actually work in your favor, but every other player is roughly dependent on the same dice rolls. If everything in a competitive game was down to "skill" then you'd be left with a game like Chess (which is commonly criticized for having a competitive "dead end" these days). Or on a simpler level, Tic-Tac-Toe, where the game is "fair" due to no randomness but where the game is completely deterministic.
Of course, fighting games essentially have "randomness" covered because it's much harder to keep track of everything going on when every single frame matters and time+positioning plays an important factor.
Not that I agree, but I think the issue Smash players have with randomness isn't that it's random, but because it can be easily exploited in one players' favor to give them an advantage that's perceived to be significantly unfair. In Counter-Strike, players at least have SOME control over RNGs like spray patterns, so it's fair. In Call of Duty, players have fuck-all control over everything and that is why nobody likes to take CoD seriously as a competitive game, because it's stupidly designed and completely broken.
Essentially, I believe it's the difference between what makes items a "fun" (if "unfair") mechanic in Smash and what makes tripping a terrible one.
I think the actual decision-making on a competitive level leads to very heated discussion (I remember hearing a few years back that there was a lot of controversy concerning how the rules for the season's competitive circuit were being decided by certain people, cries of favoritism and corruption, Smash is some serious political shit).