The interface was designed at least 20 years ago and hasn't kept up. They've been gluing on elements of newer UIs but keeping the old around. You can keep an Office 2003 style menu but also have an Office 2007+ ribbon to very quickly make it an unintelligble mess. Other contemporary Office suites like OnlyOffice handled things so much better by comparison.

Smaller elements such as just trying to do something where you knew where it was in Word versus LibreOffice is also a convoluted mess. PivotTables for example in Calc are almost complete nonsense.