The thing is that not everyone uses Garg or Sneasler, and not all Sneaslers use dire claw.
99% of Sneaslers use Dire Claw, 99.99% of Gargs use Salt Cure, and both are common enough on teams that they warrant prep, thus justifying Covert Cloak usage in some cases.
Other common secondary effects like spirit break, mystical fire or para don't matter or not even affect them so is far from being optimal
Very few people use it for the guaranteed secondary effect moves. It's more so for the elimination of rng with other common moves. During the original Chien-Pao suspect test meta, I was running a Covert Cloak Dondozo because it dealt with Salt Cure as well as two common moves from Pao in Crunch (no defense drop) and Icicle Crash (no flinch chance). It can help with other (somewhat) common moves, namely Discharge, Dark Pulse, Zen Headbutt, Iron Head, Razor Shell, Poison Jab, Steam Eruption, and of course Dire Claw.
Also, it doesn't help that convert Gholdengo loses to curse + tera water garg anyway.
Sure, but that set isn't really as prevalent anymore. That set needs Earthquake to beat Gholdengo, and I just don't run into EQ much at all. Gholdengo isn't the only viable user of Covert Cloak either; I would argue any Pokemon that can reliably heal and have the bulk/typing to handle Sneasler & Garg can run it. With Sneasler especially, Covert Cloak can be especially deceptive since it's not obvious whether or not something is holding it (unlike Salt Cure with its guaranteed side effect), so the Sneasler will likely keep fishing for hax only to be disappointed and potentially cause tilt, which I love