The problem is that some background in law is necessary when writing laws. Learning to think like a lawyer is a skill. And where a lay person will write a law that sounds sensible, the lawyer will see loopholes that can be exploited and know of contradictions in other laws.
Anyone who's worked professionally as a programmer has seen a similar effect. Non-programmers see only the success case and, perhaps, a few obvious error cases. Programmers more naturally see the edge-cases and are able to think of their users as possible adversaries. Our laws are very much like computer programs, but written in English and executed in our judicial system instead of a computer.
We don't want non-lawyers writing our laws any more than we want non-programmers writing our code. Having a politician without a legal background seems akin to a non-technical product manager. Absent some other form of expertise that's being put to use, that's usually not a good thing.
You're right that legal expertise is essential to drafting bills. But you don't need to be a lawyer--you can also hire one. Legislators have lots of expert resources available to help with drafting including the offices of legislative counsel[1] and professional staffers and committee aides.
Which is why I compared the elected official to a product manager rather than the programmer. Product owners don't have to be technical, but the best ones often are. And in the cases where a non-technical PM is the best, they're usually bringing some other form of expertise to the table.
I think scientists writing laws, assisted by the legal expertise you mentioned, would be a positive change. They have that other form of expertise. But I also take issue with the above implication that lawyers are somehow unfit to be politicians. They are, in my analogy, the technical product managers. Absent some other form of expertise, having a lawyer in that position is probably the next-best option.
Anyone who's worked professionally as a programmer has seen a similar effect. Non-programmers see only the success case and, perhaps, a few obvious error cases. Programmers more naturally see the edge-cases and are able to think of their users as possible adversaries. Our laws are very much like computer programs, but written in English and executed in our judicial system instead of a computer.
We don't want non-lawyers writing our laws any more than we want non-programmers writing our code. Having a politician without a legal background seems akin to a non-technical product manager. Absent some other form of expertise that's being put to use, that's usually not a good thing.