Question: Don't we actually want to run the shells as login non-interactive shells even on linux?
Correct, we have no choice on the non-interactive part, and as noted, non-login shells don't read anything interesting, the theory from early Unix being that since everything runs from an interactive shell which read all the startup files a spawned non-login shell can just inherit the environment which is faster than reading files again. That theory is no longer sound in these days of GUI systems.