[MisterBastahrd]

Probably because they can until they can't.

You're (a) expecting the new folks to stick around long enough to fully learn the system and (b) that they will have access to all the knowledge around the real tricky parts of the system that they have. A senior dev who has shown he wants to be there is better than 3 new recruits, all day, every day.

[JAlexoid]

I don't know where you are... but my biggest issue with the last few years, was that promotions were handed out left and right. Half of the engineering teams I worked with in the last 3 years should be demoted from being called Senior anything, considering most of them are at most 3-5 years out of college.

Just today - a Staff DBA was completely befuddled by the 5000 concurrent connections config parameter on Postgres RDS, when they had the task of making sure that the services stop dropping connections for the last month.

My previous job has a "senior" engineer that routinely blocks any attempt to move from an in house NodeJS proxy, because it would cost an extra $70 per month... and for the architectural decisions the explanation is routinely "because I like it".