Why is it? A job is better than welfare, therefore it has value. The requirement that jobs are 'renewable' is only required of the average job, not every job.
Accepting the idea that renewability is either slightly more than 1.0 or slightly less, if the "average job" is renewable it suggests most of the common jobs have to be renewable in practice.
We could argue this in another way, which is to say that the average "career life course" or let's just say "life course" has to be renewable. This is where we encounter the idea that people should be setting themselves up for promotions -- but most people don't get promotions. It's not how life works. Promotions don't address the concern for the average life course.
The main features of janitorial work is few barriers to entry. There is fair demand, but there is plentiful supply which is why it is relatively low paid.
If Janitors could only replace Janitors, then being "non-renewable" would reduce supply, increasing wage, and maintaining balance.
In reality, the main feature of Janitorial work is most people otherwise lacking qualifications can do it; i.e. anyone. So it's not true that the renewability of janitorial work has a direct affect on supply.
Instead, it depends on overall supply of unskilled labour.
I thought about this for a while after you wrote it.
In a stable setting, where there is not a large and pre-existing supply of prey, prey animals tend to have large broods. Many off their offspring are preyed upon or otherwise unable to reproduce, but just enough make it in each generation and likewise manage to have large broods.
This is not a behaviour that seems plausible with modern people. The amount of time required to educate someone prior to their entering workforce is enormous now -- we can't afford to do something like what prey animals do, with regards to work.
Where does the unskilled labor come from?
If a position does not pay well enough for someone to have a family and raise and educate two or three children, it can be said not to pay for itself: which is to say, not to pay for its renewal from generation to generation. This is the real standard by which we must consider the wages paid to those positions, like waiters and teachers and janitors, which are perennial.