Like many things in life, the answer is just "because I want to" and "because I can". Not everything has to be a linear progression or a level-up mechanic like a mmorpg.
Sometimes you want to create something out of creativity and you like seeing it done. The same way a painter wants to create something, not to show off skills, not to linearly progress onto something, but because they like making something out of nothing.
Like many things in life, the answer is just "because I want to" and "because I can". Not everything has to be a linear progression or a level-up mechanic like a mmorpg.
Sometimes you want to create something out of creativity and you like seeing it done. The same way a painter wants to create something, not to show off skills, not to linearly progress onto something, but because they like making something out of nothing.