Like others, I am having trouble putting my finger on the reason, the best I can do is to say that in Calc my attention was often on the software itself rather than the data. To use Heideggerian terminology, Calc often feels "present-at-hand" whereas with some experience other spreadsheet tools feel "ready-to-hand".
I initially assumed I simply had more experience with Excel and would find Calc just as easy if I spent more time with it. But then I tried Google Sheets and to a lesser extent Apple Numbers and was able to quickly transition from thinking about the tool (present-at-hand) to thinking about my content and tasks (ready-to-hand).
I still have to drop out of my task flow when using an unfamiliar feature in Excel, Sheets, or Numbers but these feel like momentary digressions with the majority of my time spent thinking about the content. With Calc the ratio felt reversed, with the most of my time spent thinking about the tool. (Though I'll admit I have not tried it in several years -- perhaps it has improved since then.)
I wish I could provide more actionable feedback, but I suspect that the best way to pay down some of their design debt would be for Libre Office to conduct usability tests.