This was my first time ever hearing of a .psb file so I had to look it up. Jeminy!
More specifically, PSB files can be as large as 4 EB (over 4.2 billion GB) with images that have a height and width of up to 300,000 pixels. PSDs, on the other hand, are limited to 2 GB and image dimensions of 30,000 pixels.
This harkens back to the time, comic Steven Wright boasted he had a map of the United States that was actual size. We're getting there Steven. 😊 Anyways, how would one even store a 4 billion GB file, let alone print one?! Just saying.
"Those who make large commercial SIGNS need this extra large format since signs can be VERY large, like billboards, etc. I wish PS Elements could do this also, as it would be cheaper."
Very large signs generally don't need huge sizes in pixels, since they are seen from far away. Such jobs generally require vector softwares like Illustrator or Indesign, they are not in the scope of the 'consumer' Elements. Specific professional needs are worth subscribing to pro software; after all you invest to earn money from your job.
You can submit a feature request on the feedback forum, but it's highly unlikely it will have any effect.