They struggle with it for the same reason people on the political left always struggle to understand why some people oppose new regulations: the question of whether and how to regulate commercial activities is always a proxy for deeper underlying differences in how people view the world. GDPR is just a proxy fight between the left and the right and is showing all the same characteristics.
Consider adventured's sibling post - it quite astutely points out that GDPR discussions are much more vitriolic than you'd expect for discussions of the minutiae of data handling. People who say that GDPR compliance is hard are being attacked on a personal level. He explains it as 'emotional investment' in GDPR but I don't think that's a good explanation; the people arguing most strongly for it are also those saying it's not much work, so that seems backwards. You'd expect people who put in the most effort to be most emotionally invested in it.
There's a much better explanation available: your view on GDPR is a direct consequence of your assumptions about human nature. If you believe in the existence of benign and enlightened technocrats then GDPR seems like excellent progress towards building a better world - it's extreme vagueness and severe penalties are exactly what's needed to foster obedience to technocratic elites. People who complain about this are just being unnecessarily awkward ... just be reasonable after all, and you'll be fine! The EU are reasonable so if you're reasonable too, you have nothing to fear! From this perspective, anyone who objects to GDPR or actually decides compliance is impossible must - almost by definition - be being unreasonable. What are they hiding? Why can't they just get on board; the only answer available is that they have flawed characters and any points they make about gray-area debatable things like cost:benefit ratios must be some sort of obfuscation.
If on the other hand you believe the whole idea of wise and beneficent bureaucrats is naive, then GDPR looks like a hell of a lot like a power grab by the very sort of people who shouldn't be able to grab power. Vagueness is of deep concern because it's in the shadows of vagueness that abuse can be found, and when a law is nothing but vagueness, it even makes sense to question to motives of those who created it - that's a problem because lots of self-styled Europeans have bought into the EU's utopian rhetoric and can't separate criticism of the EU from criticism of themselves and their desired future.
There's no real scientific way to prove whose assumptions about human nature are right. The USSR was a rare example of a real-life experiment in who was right and for a long time it proved the American style, conservative, small weak government is better mentality to have superior results. But that was decades ago and many have forgotten or weren't alive back then, so now rule by technocratic dictatorship seems attractive again.
As a consequence GDPR discussions will always have the same flavour as Clinton v Trump debates, or Brexit debates, or whether to restrict spending on political campaigning. They are ultimately about the same issues.