Why Is It So Hard to Develop Jet Engines?

A m c a is definitely going to be a fifth generation fighter jets with stealth characteristics and various other features. It's all going to be based off the learnings nd lessons learnt during the development of the tages fighter plane, which was a long process but still worth it. The first jet engines were developed by gen german scientists in second world war. So why is it so hard to develop jet engines? We want something that's much more powerful.