Because being an employer doesn't give you carte blanche to do whatever you want to an employee.

In the USA, it seems like it might as well. And somehow most employees have been convinced this is suitable. Whether it should is another question.

Because then less people would want to be employed and would rather stay on unemployment benefits burdening the state and the other employers? Or maybe because slavery was abolished 200 years ago?

I knew a guy who worked night shifts because he wasn't getting notoced by the shift supervisors when he went out for a smoke. There were less shift supervisors on night shifts.

Good point! Said platform should also electrocute an employee lagging behind. You know, for motivation.

See how broken your logic is?

Your counterexample is a straw man argument where the example approaches abuse.

My reasoning of why employee surveillance is fine is that there's a genuine purpose/need (measuring employee productivity) while personal privacy is retained (e.g. don't import personal information into the corporate 365 environment and personal privacy is retained). I'm not saying the metrics provided by 365 are correct, but I don't see the legal argument against it.

Read the GDPR or national equivalents in the EU and that will show the legal argument - literally that there are a group of laws defining that this isn't allowed in those jurisdictions. Whether or not you're convinced or not is of no consequence to the laws.

Can you point to a particular clause or section that indicates this wouldn't be a compelling usage (e.g. performance management of employees) of personal data?

You along with all of the other commenters have just responded with the generic assertion of "European privacy rights aka GDPR, duh." But then you don't actually support your assertion whatsoever.

GDPR is not a blanket prohibition on collecting data.