I understand that spellchoice starts at first level, and that domain should be chosen from the start, but shouldn't it be consistent between all classes?
Druids choose their archetype at 2nd level, and Warlocks effectively have 2, 1 at 1st level, one at 3rd level, so there isn't really some universal rule that is being broken here. Archetypes are just another class feature, even if they are a particularly important one.
For that matter, the term archetype isn't even a game term. Fighters choose a "Martial Archetype", Rangers choose a "Ranger Archetype", and Rogues choose a "Roguish Archetype". None of the other classes even mention the word archetype. For example, the Barbarian chooses a Path, the Warlock chooses a Pact and a Patron, and so on.
This isn't a case of a general rule which spellcasting classes have an exception to; different classes get their features at different levels.
It's not simply about spellcaster versus non-spell caster. Clerics, Warlocks, and Sorcerers are determined by major factors that are largely out of the character's hand, and not really a 'choice' once the power is obtained.
Those that decide at first level
A Cleric starts as a worshiper of a specific deity, and a specific aspect of that deity. This decision is something they dedicate their being to, and becomes an intrinsic part of the character at a much earlier developmental stage of their lives. It is what defines the type of priest they are from the opening of a story.
Warlocks don't have power at all until the power is bartered for with the extra-dimensional entity, thus the decision makes sense to be at level 1, at least for patron. Pact can wait until they prove to their patron that they aren't just a mook.
Sorcerers are using intrinsic abilities that they only sort of control, or are born with by blood. Their magic is colored from day one by background, and thus it makes sense to make this decision at level 1. It likely wasn't a 'decision' for the character themselves.
On the other side
Wizards make the decision at 3rd level because they are generalists first, and find their niche as they grow. They may already have a preference, but it isn't concrete until they begin to get much more powerful (2nd level spells).
Fighter's are pretty common and simplistic in their basic function, and many tropes associated with the fighter involve them growing over time. It doesn't take as long training-wise to become a fighter (according to old charts in older versions of the DMG), and thus the character is still slowly forming itself. A decision like Champion or Battle Master forms around the character as they grow, it is not defined from their young decisions or circumstances (most of the time).
Most other classes are like that as well. They have the luxury of time to find themselves fully. Some take more time than others.