- There is absolutely no need to start the fighter or rogue at level 1. A level 1 champion, battlemaster, and rune knight are going to look basically the same anyway
The Eldritch Knight Fighter needs to start at level 1, participating in magical culture.
The Psi Warrior Fighter needs to start at level 1, developing innate talent.
The Trickster Rogue often needs to start at level 1, either innate talent or magical culture.
Soulknife Rogue needs to start at level 1, developing innate talent.
Phantom Rogue needs to start at level 1, exhibiting affinity with the dead.
Paladin needs to start at level 1, typically a member of a specific order and sometimes a champion of a personal cause. The class is the result of a personal oath or commitment.
Cleric needs to start at level 1, typically a member of a specific spiritual community or personal calling.
Warlock needs to start at level 1, gaining magic as the result of a pact.
Bard often needs to start at level 1, developing magic as part of a college or family of Bards.
Druid needs to start at level 1, being an elementalist/weatherwitch or an animal/plant shapeshifter is quite different in concept.
Monk needs to start at level 1, typically a member of a monastic spiritual community, where the way of elements, or open hand, or shadow, are quite different traditions.
Sorcerer needs to start at level 1, developing a specific innate talent.
Wizards were everything magic in the earliest editions, but the Wizard today needs much more focus alongside other full caster classes. Focus requites level 1 character concept.
Most (all?) classes include concepts that require starting at level 1.
Ultimately, it is the players who have a specific character concept that they want to play, who decide it has to be level 1.