Huh. Now that you mention it, I don't think I've seen digital games have their prices go down permanently on Steam except on rare occasion. I guess this is one of those unspoken rule situation? Does Valve/Steam have a clause somewhere in their TOS that actively prevents lower prices on other digital marketplace?
Yes, there is such a clause. Maintaining price parity between Steam and other marketplaces is a requirement for publishing on Steam. Mind you, this applies to the base price of a game, not any temporary discounts. If you put your game on, say, GOG, and want to discount it deeper there during the holidays than on Steam, you can do that. But not permanently discounted, of course, because that would be interpreted as trying to undercut Steam.
Valve does also allows you to freely generate as many Steam keys as you want, which you can then sell via official key sellers like Humble, Greenmangaming or Fanatical, or even give away, and which Valve does not make a cent on. These key sellers also charge a commission for selling via their store, but generally lower than Steam's 30%. Iirc, it's only 5% at GMG and Fanatical. While you can't sell your game at a cheaper base price on those stores, same as GOG or EGS, you can offer discounts. So the trick is to discount your game on those key seller sites a lot of the time, but not permanently. So you're technically not undercutting Steam, you're just offering a game on sale. And because of the lower commission those sites charge, you make the same money or more as you would on Steam.
This a loophole, of course, but one which Valve seems to tolerate, because most people aren't aware of these official key sellers and thus the majority of a game's sales will still come from Steam itself. And the keys are Steam keys, so customers will still need to engage with the Steam ecosystem, including any DLC or MTX they may then buy through Steam, and which Valve makes money on.