I'll throw another possible explanation into the mix: the gap between "senior" and "staff" can be quite large in some places depending on the threshold for being titled "senior". I've seen places where promotions to senior is not uncommon for someone 3-4 years out of college but with no staff engineers under the age of 30, and only a few under 35. Without any other granularity of IC roles, the only options for a young engineer are to be "stuck" for a decade at a role with a title so coarse that it doesn't really convey seniority (which tends to mean that the actual "seniority" of an engineer is implicit institutional knowledge), swap to a manager role, or go somewhere else with a more explicit career path for ICs.

The number-based systems at the super large tech companies can be hard to understand from the outside, and they might introduce potential confusion when trying to calibrate between different companies with incompatible systems, but they at least allow for a more clear trajectory for an individual contributor (although whether this actually determines how promotions happen in practice is a crapshoot; internal company politics can't be fixed by nomenclature). I don't think that software engineering is somehow different enough from other professions that levels can't be classified with descriptive names rather than numbers, but clearly _something_ needs to change when we can't even all agree on what "senior" and "staff" mean.