Because we don't have any efficient way for states to figure out exactly how much people were making before. Lawmakers have been on a campaign to defund, stifle, and keep unemployment insurance hard to access. This has resulted in ancient systems (even by government standards) that makes it very hard to figure out how much people make.

I cannot find the specific reference for it, but I believe I heard people saying some of these systems were having a hard time paying out over $1,000 a month because they were so old and outdated.

This is the case of "don't let the enemy of the perfect be the enemy of the good" that is repeated a lot in policy making. Sure, we could create a more perfect solution, but what are the costs associated with crafting that perfect solution under the specified given time frame?

https://www.cnbc.com/2020/06/19/outdated-technology-may-slow...

https://www.marketplace.org/2020/06/05/when-does-the-expande...