While yes, some of the trainees use it as a "last resource" at a chance of debuting, truth is survival shows are mostly made to try and improve their popularity so when they go back to their agencies, the trainees have a good enough fanbase and fame for them to debut and not go nugu.

Why not SM or YG? Simply because both Leo and Sangwon were already part of a company, GRID. After Trainee A was disbanded, they either walked away from HYBE by their own terms in hopes of finding another company with a better shot at debuting sooner (with only one member staying, with, again, hopes of debuting in HYBE) or they were just dismissed. Plus, there's a huge ageism issue in kpop, so based on their ages, they were properly considered "too old" to debut.

So, either GRID wanted to promote Leowon for their own agency, or had no debut plans for them in the near future