Because 90% of buyers don't know any better. If it lights up and maybe moves a little bit, it's a lava lamp.
I did see some people at a Fry's Electronics wanting one, but complaining about the cloudiness of the display models. I told them to look up Mathmos if they want a good lava lamp, told them a tiny bit about the quality and where to look.
I stopped in my local Spencer's last week, not a thing with the Lava logo... I asked the manager if they were discontinuing to sell them & it's a huge yes, for the Cincinnati tri-state area.