The main reason is: Stripe has 21 products, each of the product has a clear pricing, but it's unclear how many of the Stripe products you're using. Ans each pricing is negotiable at some point.
Here are our notes on this topic: https://www.getlago.com/blog/we-crunched-the-fees-of-stripes-21-products
Disclaimer, I work at Lago (YC S21) and we build an 'Open Source Billing API'(we partner with Stripe and other PSPs).
We also came across this past HN post: worth reading on this topic: https://news.ycombinator.com/item?id=25072783