Why not just use Asimov's three laws for model safety and alignment?

Because those laws were explicitly written in order to write interesting stories about how those laws can be subverted, how they can go wrong, and what loopholes exist in them.

This is the equivalent of asking, "why don't we build the Torment Nexus? You know, that thing from the classic science fiction novel 'Don't Build The Torment Nexus'?"

Because they're science fiction?

Kind of like asking why we don't just use warp drive for space travel, or transporters instead of airplanes and hyperloops.

My less sarcastic answer: Because there's no "we" to decide such things. The companies that own what we're calling AI have little to no incentive to build in the "three laws," should we get to that point, which I doubt. If we had useful robots the military would use them and they wouldn't want robots that won't kill people. If we can imagine robots and AIs with the three laws embedded in them we can also imagine robots and AIs without those prohibitions, and people hacking them. The bad actors will always drive the arms race.

Say AI used the the 3 laws today. You can still have cloned people online, faked calls stealing money from companies...

Google recently pulled their AI image generator, it wouldn't have solved that issue either.

They don't really solve the issue.

What about the zeroth law? Nobody remembers that Asimov wrote another law in his works.