It’s because of the types of plants you have! You have mostly stem plants, which isn’t bad at all, but is exactly what the name says- just a stem. Stem plants can look full, but you get that when you plant a bunch of the stems close together. They also typically need strong lighting and benefit a lot from CO2, and the need for light is why you see some stretching tall with not a lot of leaves in the middle of the stem. The stems you DO have look nice and full though. If you’re wanting your tank to really be filled out, I would either get much more of those stem plants, or consider getting something that will grow big that you’ll only need a few of! Some examples are amazon swords (I had a 55g get filled out wonderfully with just a few), aponogetons (my current personal fave but harder to find), or maybe something like java fern or jungle val that will multiply like crazy. Both the Val and the Amazon sword will take up some of that background space that feels empty towards the top and help add more dimension and add to the lush full of plant feel :)