Since it's perfectly okay, why only do it rarely? Since it's perfectly okay, why do folks need to be so clear that they only do it rarely?
Because, as Merlin noted, it is a tool that works best when used only rarely.
Also, because there seems to be a big bugaboo out there of GMs who as general policy drive the game to highly specific predetermined results. It then seems prudent to point out that the tool is not being used for that end, but instead to make only occasional edits for specific instances.
And another one: Since it's perfectly okay, and right there in the rules, why keep it secret from the players?
I'll repeat - for the same reasons that I keep the stat blocks, maps, and room descriptions from my players, and why I avoid extended rules-debates in the middle of play. The players don't get to know what's happening behind the curtain during play, and the game needs to progress smoothly.
My viewpoint is sorta this: If I'm doing my job well as a GM before the dice are rolled, I should never need nor want to fudge.
Mostly true. I think there are occasions where things go wrong that aren't clearly connected to the GM's ability. Those occasions are not particularly frequent, but they do happen, and they will happen in any game that includes random elements.
And sometimes is seems to me that players are themselves a random element
But when I have to do that, I take it as an indication that either I didn't do my job well before picking up the dice ... or I should be resolving things differently, whether without the dice or with a different system.
I'd not call it an "indication". More like evidence. When evidence stacks up, one should look at it and see if it indicates a pattern. If there is a pattern, one might want to look at changing things to eliminate that pattern going forward.