I really doubt any intelligent, experienced, competitive game designer would ever agree with you on this point. The mechanics are there for 'fun.' That's how every mechanic that doesn't focus on player skill comes about.
For example, the design team felt League needed a draw for new players to play it in early, early beta. RNG and arbitary comeback mechanics appeal to casuals. Then when they get people hooked and it becomes popular, they begin appealing to the esport community by phasing out the anti-competitive, lazy bull**** they introduced earlier in favor of promoting consistent play instead. It's definitely possible to make comebacks more plausible, and snowballing less prominent without rewarding failure, and have a fun, interesting game at the same time. That's the challenge.
No dangr, that is incorrect.
TBH, I admit that a lot of this analysis is lifted directly from sirlin who had a massive influence on the way I think about games and gaming in general.
From a competative standpoint slippery slopes are bad, because they decrease the game's ability to select for the better player over the course of a game because if a person who makes 50 mistakes of about equal calibur versus a person who makes two mistakes of an equal calibur, who should win? In games with a significant slippery slope, it's not a matter of who makes the most mistakes, it's a matter of when. Which means if those 2 mistakes are made early game, the person who made them is probably gonna lose even if they played a consistently much stronger game at every other point. (this of course doesn't account for odd clumping, which is why sets exist and double elimination tournaments).
Of course this isn't true of games where a slippery slope is not an issue since there is no less access to power, you nearly need to make up for your mistake. In this case, the the only major concern to consistently select for the better player is enough tests for odd clumping of mistakes to rarely be an issue.
As an example, one of the best mechanics to ease comeback to deal with superior resources due to the slippery slope effect is parrying in SF3. Yes, it is accessible to the player in the lead, but being at low enough health means that your block isn't available to you because chip damage will kill you. Parrying means that you can always win as long as you're not in a combo and if you predict your opponent well enough, this encourages consistent superiority over early advantages. Even then, the slippery slope in fighting games in general is relatively low, games with too powerful defensive games exempted.
League is a game where an enemy getting a material lead early significantly decreases your ability to obtain material, furthering their lead, so an earlier mistake is far more important then a later one of the same degree. It is a game with a very significant slippery slope, an issue that should be addressed.
So comeback mechanics appeal to competitive design and casual design for different reasons. For competitive design it's important if and only if the game has a slippery slope, to increase the game's accuracy in selecting superior players.
Making random events core to gameplay isn't even on the same planet as far as balance decisions. From a competitive standpoint there is no reason to include a RNG and you comparing the too is a bit silly.
edit:
Which is why you make items cost more gold. If one or two kills does not get you an item, then the weight of the mistakes is much lower. Consequently the lane comes down more to who is better at their champ / the MU rather than who simply has more items. If Shyvana scores an early kill and comes back to lane with you at dorans + pots and her with a wriggles, you're more or less already done in that lane. Increasing the cost of items and putting a gold loss on death would increase snowballing for consistent strong lane play, and reduce it for morons that simply pick characters that scale better and killtrade all day.
You don't see a Weaver getting Radiance from two kills.
Hmmm, actually you might have something there. Not having an item advantage until you are able to consistently dominate the lane might substantially improve the issue with snowballing, of course there are other implications (like decreasing the value of champs who are inherently worse in lane but bring more to teamfights). Heh.
Honestly though I think that adding losing gold to this would make the game too passive.