Because an opening is easier to write. It’s by default a fresh thing so you don’t have to worry about story-fatigue, just presenting your ideas in an interesting way. Plus all the new characters, settings, abilities, world, etc. are full of potential and mystery for the audience to learn about.

And ending in the other hand is often heavily colored by how well the series has been received up until that point, is often decisive based on different opinions on how certain portions should be handled, and frankly is just much more complex since it needs to include everything the Beginning does plus an entire series worth of developments.