I ran into this on Simple Mobile. Just seems so wasteful, just a business rule, no technical reason for this. Of course it's a revenue opportunity, buy a new sim, send the old one to the landfill. Just my opinion and it is just a little plastic chip with metal in it, but these mvno companies could be a little more conservation minded.
The reasoning behind this I believe is to prevent telephone number cloaking, a huge problem for the cellular industry years ago. Every SIM card has a unique identifier number associated with a telephone number for single use.