It’s hard to fathom why this would be the case. Isn’t creating learning materials mostly an upfront task? Obviously you want to update your materials and fix mistakes. And perhaps you sometimes have a new idea that necessitates an update or rewriting things. But creating lesson materials honestly seems like it would scale very well.
I think about reading language learning blogs and forums and people would sometimes recommend favorite resources that were from decades ago. I’m sure something was lost in them not being totally up to date but honestly, persistence is way more important than currency, let alone “having a massive amount of content.”