Because the spread of disease or alien organisms is a very real threat. On earth we have multiple historical instances where an infected shipload of people has gone on to infect and decimate populations.

My locale lost 1/4 of its population to such an event when a [New Zealand ship discharged sick passengers in 1908.][1]

So it makes sense not to always trust the pod's inhabitants and investigate before opening the pod.

It's not a spaceship you can navigate and fly to a habitable planet a few light years away and survive a crash landing. It's an escape pod, it just gets away from danger, send out mayday signals and hopes for rescue before its life support and supplies run out. So if it isn't rescued then you'll die anyway. Opening the door won't keep you alive.


  [1]: https://en.wikipedia.org/wiki/SS_Talune