Because it's the rules. Initiative is a part of combat, but it happens before anyone can possibly attack anyone else. It simply isn't possible for it to happen after someone attacks, because the very instant someone so much as thinks about attacking, initiative is rolled and that person could be going last, having never attacked. Again, it's simply not possible to roll initiative AFTER someone attacks. Even in a surprise round, initiative is rolled before a single person attacks.
You're mixing up fictional events with actual events at the table. Players at the table in the real world roll initiative to resolve who goes in what order each round in a fictional combat that has already been established to be taking place in the fiction. The way this is established is usually that one of the players or the DM declares an action for a character that s/he controls that requires resolution in combat rounds. Part of the resolution of that and subsequent actions declared for the participants is to roll initiative to determine the order of resolution. So first it's established at the table that combat is happening in the fiction, and only then are the combat resolution mechanics engaged, including the rolling of initiative.
I don't think it's a good call for a DM to call for initiative because a player or a character is thinking about attacking (it's unclear which you mean, but I don't think it matters), because I can imagine plenty of situations in which someone is contemplating making an attack but decides not to, and rolling initiative in that case would have been a pointless exercise. That's because it hasn't been established yet that combat is taking place.
They are surprised by the appearance of the other side. I have walked around a corner and been surprised by a squirrel that I didn't know was there and run up a nearby tree. I have rounded a corner and been surprised by a TV sitting on the sidewalk that I wasn't expecting to be in my path. Are you really going to argue that the TV was attacking me?
And no, if both are sneaking, then both can be surprised by RAW as neither side noticed the threat before it was upon them. Initiative is rolled and they do nothing in surprise, then attacks begin in round 2.
Look at the examples under "Surprise". Adventurers surprise bandits by "springing from the trees to attack them." A gelatinous cube surprises adventurers by engulfing one of them before they notice it. In each case, the unnoticed party surprises the other by attacking before being noticed. The fact that the resolution of those attacks at the table awaits the engagement of the combat resolution mechanics in no way means that offensive action hasn't been committed to in the fiction. If not, then why engage the mechanics?
The word typical simply means how combats typically work. The atypical portion is not defined, so is as likely to include what I described as it is to include more than two sides.
No, "atypical" combat that isn't "a clash between two sides" is still defined by other statements about combat. It's a set of rules for "characters and monsters to engage in combat," not, as you have described, for them to have chance meetings in dimly lit hallways after which they decide how they feel about each other. The participants in combat are described as taking part "in a battle", not as deciding whether they want to be a part of a battle after initiative has been rolled. Combat, both typical and atypical, is just that.
Whether or not you think it's worth distinguishing as a separate category, it is in fact a separate category. You cannot determine what direct opposition is, without knowing what indirect opposition is. Opposition is always one or the other, and only one uses the contest rules.
My point is the distinction you're making has no meaningful difference in actual play. I'm speculating, but I don't think the designers intended for the word
direct to hold as much weight as you're giving it. Compare this with the language used in the "Contests in Combat" sidebar where there's no mention of "direct opposition". All that's required for a contest under that description is that prowess is pitted against prowess. Dexterity is a form of prowess.
You're stretching things here, but regardless, initiative is not direct opposition and never will be.
Yes, it is. It represents the directly opposed efforts of you and your opponents to do whatever it is you're doing on your turns before they do, and vice versa.
No, initiative isn't finding out whether you are successful in swinging your sword before I cast the spell. Do you know why? Because after you roll initiative, you might change your mind and push me, or grapple me, or run away, or a number of other things. Winning initiative doesn't lock you into an action, while you are locked into your action as soon as a contest begins.
The "action" you are locked into by rolling initiative is trying to do whatever it is you do on each of your turns in combat before your opponent does whatever it is they do on each of their turns. The sword swinging and spell casting were just examples.
Going by his tweet, all we know is that initiative is not a contest. Period. Nothing of his motives are given.
Right, so my speculations as to his motives are just as valid as yours.