Because they need to understand medicine when they make drug (the prescription kind) or other medical related laws, or chemistry when they make environmental regulations? Please. Only a superficial understanding of technicalities is required to legislate sufficiently effectively, understanding that can easily be supplied by staff who make high-level summaries.
They don't have to have an intricate understanding of medicine, but they should understand the issues well enough to be able to make informed decisions. If they do not understand, then they should bring in experts. For example, if you want to make legislation regarding a certain vaccination, then you should understand how vaccines work at a high level and the concept of herd immunity. Otherwise, you have no business writing laws to regulate it, period. The people writing this legislation don't have an equivalent level of insight into how the internet works.
In this video, we had a lot of people proudly proclaiming that they didn't have the slightest clue about how the stuff they were regulating works, made terrible and inaccurate apologies, all without bringing in experts who could inform them as to the implication of the law. To add insult to injury, rather than calling them experts, they used derogatory language to put down the very people who knew what they were talking about (lets be clear here, some people use nerd as a badge of honor, but that was clearly not how they were using the word).