I think it comes down to 2 main reasons, and some members of the libreoffice suite definitely do a better job than others.
Comparability with MS Office, it's really difficult to use these programs when you can't reliably collaborate with people using the de-facto standard office software. Impress is exceptionally bad at this.
User interface clunkines, the ribbon ui Microsoft uses in modern office versions is really nice, and makes finding the actions you need really easy. This is coming from someone who used office 03 and 07, it's not just a learning thing, it's a better design.
These issues are definitely a bigger deal on some parts of the suite than others. I've found Calc to be a solid replacement for Excel, but when I'm making spreadsheets I'm not fiddling with complex formatting at all. Impress is on the opposite end of the spectrum. It has horrible comparability with PowerPoint, and I need to get things looking just right when I make a presentation. It's difficult to find even basic formatting options. I could probably solve the usability issues by reading a few tutorials, but the comparability issues hold me back from putting the time in, since I have no idea how a presentation will look when someone loads it in PowerPoint anyway.