Cupax—
The exact question you asked is, “Why the developers don’t fix the most basic and annoying bugs in LO instead of spending time adding new features very few people actually need?”
Firstly, I believe your premise is false. But leaving that aside for a moment, there is a very simple answer (if your premises were true): because they want to.
The developers aren’t paid. They are volunteers. They program LibreOffice because they want to.
If you don’t like it, you don’t have to use it. No one is forcing you to. No one is getting paid because you use LibreOffice. You can go on using MS Office forever and no one who programs LibreOffice will lose any fraction of a cent because of it.
Not only are you free to use it or not use it, you are even free to work to improve it. Pick up a book on programming and dive in.
If it makes you feel special, you should know that even without any experience in coding at all, you can get a starting pay exactly as high as the very best, most expert programmers get for their work in developing LibreOffice. 
Now I want to address your premises: 1. That “the developers don’t fix the most basic and annoying bugs in LO.” 2. That they “spend time adding new features that very few people actually need.”
First I’ll address premise 2. LibreOffice has been around for years. MS Office has been around for years. People have been successfully using them all that time. They have a LOT of features. Most people who use MS Office or LibreOffice need very few of the features available. Many of the features already present are needed by very few people.
I am familiar with the issue you refer to on the screen display. In actual fact, it is not that big of an issue for me, or for anyone else I know. In my experience, the vast majority of users of LO and MS Office are so basic and simple in their requirements that they just want something to write words on a page that they can print, and to put words in a grid when they want to. I know plenty of power users also and not one of them would care about the occasional glitch in display, as long as it doesn’t corrupt the file data or mess up the printing appearance.
Everyone here answering your question is just a user. I won’t say a user “like you”, because most users of this fully functional, professional level software that is provided free, with no obligations, are not so hypercritical of the developers who are offering assistance with no return expected.
Possibly you are one of those people who feels that the world owes them a living, and that if you don’t feel like working then other people must make sure that you have food to eat. And if you don’t like the food, then they are obligated to stand there and listen to your complaints also, right?
This forum is not a complaint box. Get constructive or get out.
—Summer Glau
