I am curious about that as well, because I heard that one often and whenever I ask: "Like what?" it is either some obscure function that you could build yourself with a few lines or it is crickets and a: "Don't know, I just like it better".
Note: I am not saying there are no true and valid differences one or another person could really rely on. But having worked in IT-support I know that not few people like to give pseudo-rational reasons, when in fact it is often more about feelings. I don't say that feelings/look/design is irrelevant: "I like the design of Excel more", is a totally valid reason for choosing it over Libreoffice. But in the end many people pretend there is more to it. I have yet to encounter a situation that I couldn't resolve using Libreoffice Calcs features (ignoring insane scenarios where writing a well tested script or using an actual database would be the rational option).
I would make a difference between "design" and "a lot of small and convenient functionalities that makes your life easier".
People prefer gsheet to calc not because of one big missing functionality, because as you say, you can always find a way. It's more that excel/gsheet makes your life way easier, and saves you a lot of time.
I use both gsheet (for work) and calc (for personal stuff). I really want to give calc a chance so I accept the excess time it takes me to use it, but I understand that some people don't. And I would not say that it's related to "feelings", it's more a problem of "ease of use" and "doing things fast".
I agree that design (rather than functionality) holds Calc back, but chalking it up to personal preference may prevent the Libre Office team from increasing their user base.
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.
Note: I am not saying there are no true and valid differences one or another person could really rely on. But having worked in IT-support I know that not few people like to give pseudo-rational reasons, when in fact it is often more about feelings. I don't say that feelings/look/design is irrelevant: "I like the design of Excel more", is a totally valid reason for choosing it over Libreoffice. But in the end many people pretend there is more to it. I have yet to encounter a situation that I couldn't resolve using Libreoffice Calcs features (ignoring insane scenarios where writing a well tested script or using an actual database would be the rational option).