They've played RPG's their entire life and have a strong understanding of mechanics. Game design has a lot of overlap.
This is a vast over simplification of game design. Game design is more about player perception and expectations than the mathematics of underlying systems.
We can see this in an example. Josh Sawyer, a lead designer for Obsidian Entertainment, spoke at a convention, its on youtube, about an instance of this. Where he talks about how a player base found a gun, in one of their games, as under-powered. In a following patch, Obsidian changed only the sound effect. After that single change to the sound, players thought the gun was much better. It was not, it was the same. The perception of the sound, made the players think the math was off.
We could see evidence of this in the playtesting for 2024's rule revision, as well. Where on multiple issues, notably with Paladin and Druid, player's voted heavily in favor of "cool" and away from a mechanical balance. Such things as the limitation of druid wild shape options to three generic statblocks was objectively more balanced, but wildly unpopular. Player perception was misaligned with WotC's mechanical goals here.
We can dwell into perception from other angles, like how game developers largely avoid nerfs because buffs are better received. Why video games like X-com "fudge" the math, because player perception of random isn't random. Or how players react to different ways of increasing difficulty.
Experiencing mechanics provides little outside of perspective. And when your understanding of mechanics is solely with one game, even that benefit wanes into irrelevance.