That same loose hand is what can often make play less certain for the players. 5e was designed loosely so that GMs could plug the gaps. And while some folks see this as a strength because it gives them freedom to GM how they like, it also leads to uncertainty in play process and hides a lot of information from the players that it perhaps shouldn't. At the very least, it should talk about the impact that hiding information from the players has on play.
The flaw in your argument is that it incorrectly attributes the issue to the “loose hand” itself, when the real problem is a breakdown in communication.
Yes, mechanics can be an effective and terse way to convey how things work, whether it's what a character can do, the abilities of a monster, or how a campaign is expected to function. But that's just one way to communicate, and it’s not always the best one. People vary in how they process and understand information. What works well for one player may be opaque to another.
Moreover, using game mechanics to communicate how a campaign works carries an implicit assumption: that the rules are not just descriptive but prescriptive. Many people grow up believing that the fair way to play any game is to follow its written rules. If something isn’t in the rules, they may assume it’s off-limits or unfair to invoke.
This can lead to problems when designers rely too heavily on mechanics to convey expectations. Overly rigid application of those mechanics, especially by players or referees who interpret the rules as a complete representation of the world, can actually obscure the intended flexibility or tone. If that’s intentional, fine. But too often, I don’t see designers consider this consequence.
As for me, this concern is why much of my Living World sandbox framework is presented as advice, not as a rigid system of rules. I want to give referees tools, not prescriptions, because communication, not mechanical enforcement, is the key to clarity and trust at the table.