Because that knowledge is worth thousands to tens of thousands of euros in lawyer time. And you're still not guaranteed to get it right or be covered.
Your example is like saying that everyone that wants any kind of job should know multi variable calculus. When people protest that that's putting too much of a burden on people, you bring up that you got a job just fine, because you learned multi variable calculus in school.
Their example is like saying if you want to open a restaurant you better take the two day course on food safety. Equating GDPR compliance with multivariate calculus is just a gross exaggeration. Yes there are risks, you get those with every venture you start. You're pretty well covered with the technical due diligence we as a sector should have put upon ourselves in the first place and you can externalise the rest easily, just like people do with many other regulations like taxes/finances.
We should really separate the protection of scummy business models and down to earth stuff like data takeout / account deletion and transparency as to what companies do with user data. The latter is neither rocket science, nor should it be particularly hard for any startup that's over the "my company is a fancy slide deck" stage.
But it's not just that. Read the rest of the thread how much time and effort people had to spend at various companies for compliance. It's not just about data takeout and account deletion.
Your example is like saying that everyone that wants any kind of job should know multi variable calculus. When people protest that that's putting too much of a burden on people, you bring up that you got a job just fine, because you learned multi variable calculus in school.