Because I want them. Last count was 27 tanks. But I've bought more since. That's just the Bella, Elegance, Unique, and JonBoy's. Every tank is a different color with a Bedazzled glass tube. I also collect custom made drip tips, designer Ego batteries, and mods. I have them all because I enjoy them. I also use them all. I have a juice box full of flavors that I like to vape daily. I don't eat the same food for every meal so why would I want to vape the same flavor all the time or look at the same color of tank or battery. Every flavor has its own tank/mod combo set up and ready to vape. It's convenient and I quite enjoy it
I'm sure you have something that could be considered excessive. People buy what they enjoy. Some have the bare minimum for vaping. Others, like me, enjoy buying a variety of items and trying new things. It's a hobby, not just a way to stop smoking. Even if I decided to stop vaping I would still enjoy my tanks.
The same question could be asked about everything that we buy.