I have core i7 desktop and would like to upgrade past Catalina

I customized my iMac at initial purchase with core i7 even though it was late 2013 Desktop, it would seem that I would be able to upgrade past Catalina, but that is not possible without purchasing another iMac desktop, it seems so unfair that I cannot upgrade. I still have over 1TB available on my fusion drive. Can someone explain why my desktop is not eligible for upgrades