If you don't stamp on the revs the engine will die letting the clutch out revs drop to 0 and you can't restart engine even with buttons, with auto clutch revs stay at 12k and restarts.
Yep, you can certainly stall the engine if you don't have a few revs when the clutch is dis-engaged, just like real cars. However, you should still be able to restart it. In GTL, I just hold the starter with the throttle depressed a bit until it starts. You still need to have it in neutral or keep the clutch engaged when starting though. Try it without hitting the Ignition switch, maybe you are turning it off. I think in GTL it stays on when you stall. Not totally sure though.
so why are there H-shifter and clutch pedals in most sim equipment if they don't really work for us
Yes, unfortunately, it is not fully replicating a true old manual gearbox that needs a clutch. I don't think any Racing Sim replicates this (yet). However, you can still use the Clutch for controlling starts by raising the revs with the clutch engaged and dropping it on "go". It does help get better starts, especially compared to using autoclutch. You also need to use the clutch if you crash or spin out, to avoid stalling it.
I think some people also use the clutch at critical times in a race to keep some revs when it helps, and avoid wheelspin , some cars can be touchy when downshifting and upshifting to low gears eg. Capri comes to mind (Did you see the video of Rupe and I doing a synchronised pirouette
).
The H Shifter isn't exactly realistic since you can still shift without the clutch, but it still provides much more realism than using Sequential or Paddle, especially with the old GTL cars.