Because somebody has to make them. Unfortunately LibreCAD doesn't support ttf-fonts.
There is a tool to convert ttf's to Librecad fonts. From a thread with forum member jep1955: "Concerning fonts, in the main LibreCAD program folder there is a ttf2lff.exe. With this you can turn a windows ttf font to a LC lff font. But it only makes an outline. If you take a quite "sticky" font it might look acceptable. I have tried it out prior to writing this and got an error "freetype6.dll not found", but I remember to have had it working some time ago. You may play with this."
Jep1955 has done it:
http://forum.librecad.org/FW-Dimension-styles-tp5715663.htmlHe wanted a nice architectural font, too.