[PebblesHD]

It does suffer slightly from looking like it was developed in the 90s. The UI kit it uses by default looks very out of place on most modern platforms and despite their being other UIs available, most end users will look at the default and choose something else, likely office. They need to invest some time in modernising the human interface parts of the suite.

[einpoklum]

Which Desktop environment are you looking at? The "UI kit" is different on different platforms (the idea is to use native widgets)

If you mean the fact that it's not ribbony, with a lot of unused empty space between widgets and with mobile-phone-like aesthetic - the rejection of that style is intentional. Microsoft went that way, and hurt usability; LibreOffice has not. Although some argue emphatically that it's the way to go in order to attract new users.

For those who like a more ribbon'y UI style - it's available as an option. On the menu, choose "View > User Interface..." TBH, that's not fully-developed and I don't like it much.

One other aspect of modernization is that there is a dark mode, which has improved a lot over the past couple of years: https://itsfoss.com/libreoffice-dark-mode/

but if you have specific suggestions/rants - UI/UX discussions are always ongoing, albeit slowly given the immense size of the project and the amount of dialogs and UI/UX aspects to be considered and improved. Try the "LibreOffice Design" channel on Telegram for example.