I make no judgment as to a DM's particular ruling in this discussion. What I'm factually stating is that a player is not entitled to any sort of check for any action he or she has his or her character take in the game. A check is used to resolve uncertainty that the DM establishes. If the DM says that the orc can see the halfling despite the halfling's attempt to hide behind the human, then that's what happens - the DM narrates the results of the adventures' actions. If the DM says the fictional action of hiding behind the human has an uncertain outcome, an ability check is called for.
A player has no recourse to the rules which serve the DM who brings them into play as needed. A player only has recourse to the fiction. It is, of course, reasonable for a player whose character is in an exact or substantially similar fictional situation to expect to make a check for a given fictional action (or to succeed outright or fail outright) if he or she has done so before. But players are not entitled in any way to make one without the DM's say so. In fact, if you're a player, you want to strive for auto-success. Asking or demanding to make a check is just asking for a chance to fail!
If a player has an issue with his or her DM's rulings, that's a discussion to be had outside the context of the game.
I agree with you in theory, but it appears we disagree on where we set the bar.
For example:
A player who wants to walk across the surface of a lake without magical aid? Impossible. Will automatically fail.
A player who wants to jump off a 25 foot roof, into a haycart, while unlocking a set of leg shackles on the way down, with one hand. Make a roll. With disadvantage. Target number is 35. (Probably won't make it, but the player should realize that when they come up with such a hair brained scheme. On the flip side, if they do make it, they will talk about it for years).
There are many game systems which use 'complications' as a mechanic. So, while auto-success may be desirable from a player perspective (or for mundane, easily completed tasks), adding a random factor adds to the storytelling. Also, I feel it is important to give the player the DC after they describe their action, but before they decide on actually doing it, that allows them to modify the action if their expectation was different than my ruling.