I know that FFXIV, at least, does have actual constraints on supply. Square-Enix is having trouble getting more server hardware at the moment, so they can't easily add more instances.
"Since July 2021, in order to accommodate the significant increase in the number of new players, we have been working towards building a new logical data center and adding new Worlds. However, due to the effects of the COVID-19 pandemic, we have not been able to make any additions due to suspension of semiconductor production at manufacturing factories, decrease in production quantities, as well as a shortage of raw materials. Although we are investing much more money than usual, our attempts to alleviate the ongoing congestion situation are being delayed due to the physical shortage of server equipment."
https://na.finalfantasyxiv.com/lodestone/news/detail/e738898...