At Google at least, if you stay low level for too long, you get fired. If your team is full of low level people maintaining a project that's stable, you need to invent work to justify your existence.
Several years back, you had to also eventually get to L5. This was more affected by promo culture, but was also largely unenforced, which is why they get rid of the formal requirement to get to L5.
I'm unsarcastically glad that you had a different experience at Google than me. This was not my experience, nor the experience of anyone else on my immediate team.