Is there a reason you don't buy it? It might have been, and most likely, that they made the spellcasting rules first and then wrote the wish spell. They probably at least somewhat thought about what a spell's "requirements" are, seeing as they had to type the spell out. It's even possible that some of the things that don't have the word "require" used to until the found a balance issue with wish. Or they may have added the word "require" somewhere where they thought it would synergize nicely with Wish.
Or, more likely,
as was one of the stated goals of 5th Edition, they simply used natural language and counted on the DM to make rulings and apply common sense in adjudicating.
Here's the thing- the "everything is a requirement" approach that leads to being able to cast
sequester on the Abyss doesn't make any sense. It doesn't make sense because it places
wish so far above any other 9th level spell that it's ridiculous. It doesn't make sense because it allows for nonsense, such as casting
water breathing on a target that is neither a creature nor something that breathes. Or for casting
sequester on the Abyss. It doesn't match up with the precedent that
wish is just another 9th level spell- and I find the argument that the spell is in the game not to be used by pcs to be pretty flimflam; if that was the case, why would it be in the
Players Handbook?
If there was an explicit in-game definition of what "requirements" are for casting a spell, I imagine we'd have seen it quoted by now. But there isn't.
In my judgment, in the context of
wish, the bottom line is this- you have already cast the spell (
wish), and if you use it to ape another spell, then you don't have to cast another spell, because
wish is doing the work. I would consider requirements to include things necessary to cast the spell, which are almost entirely components; but sometimes, in certain cases, might include things like having to cast it in a certain location, at a given time of day or year, under the right alignment of stars, with good intentions, etc.
Does the spell say, "To cast this, you must ______"? If so, that's probably a requirement.
Range, duration, targets, attack rolls- I think it's a real stretch to try to interpret any of these as requirements for a spell. I certainly wouldn't include them.