I believe Calm does offer monthly subscriptions (https://www.calm.com/subscribe). Although I do understand that a lot of companies are starting a trend of offering yearly subscriptions, there can be many aspects as to why companies would choose to go for such a pricing model. Any company using a subscription based pricing model would want to make the customers stay for the maximum months possible, basically increase the lifetime value of the customer, maybe the pricing the product per month would be low and then the companies would have to make sure that customers keep subscribing for every month inorder to make maximum profits which also depends on how much is the Customer Acquisition Cost. Another aspect would be where the company knows that customers won't be able to see great results in a few months but the product would be very valuable on a long run, in such cases the companies would probably give 1 month free trial and then a yearly pricing.

I run a productized service (http://draftss.com) where we create unlimited designs for our customer on a monthly based subscription. We create multiple options for each design task that we do for our clients and thus we have a lot of leftover designs. We have been thinking to give away all of these leftover designs for a yearly subscription or maybe even lifelong subscription.