Really?? Key cards??

I see what you mean, but had they been able to open doors using voice/fingerprint/eye recognition etc., it would've changed the entire story. Caleb would never had been able to reprogramme the system as it would have been impossible for him to replicate Nathan's voiceprints/fingerprints. Unless, of course, he knocked Nathan out and dragged him around the building to scan his fingerprints/eyes but that would have opened a whole new can of worms. The point with stealing Nathan's keycard to reprogramme the system was supposed to be secretive and this wouldn't have worked if he had to drag Nathan's body around the premises. I think it would have just caused unnecessary complications.

Yeah dude, seriously. Added to that, he does not have any other measures other than that stupid card.

voice detection - yeah that's a great idea.

until ava synthesizes his voice and can open whichever door and go wherever it wants, anyways. other biometric devices aren't too hard to spoof either, and once they are then they're broken forever.

if nathan drops his card in the waterfall, then he just uses the one stashed under the welcome mat by the front door. 😀

My thoughts exactly

The whole film was a bit overrated

I just thought the same! I think the movie is great - but the keycards are the weakest element. I mean Nathan invented a robot who can read lies in ones face. But to turn on his phone, computer, or open a simple door in his house he needs a key card?? there are anyways cameras at every door. Its especially a pity as the whole plot only works because of these keycards. After all it's a movie about artificial intelligence... There would have been for sure much more intelligent solution to trick a system which reads identities.

They thought about it long and hard, but they needed something that Nathan could "drop", so that Caleb could alter the lock down procedure.