Ask HN: Every SaaS startup needs this but no one built it yet, why?

I just want to get a subscription product out there fast, not spend three months building yet another user registration database with oauth and payments. What I really need is something like shopify - but allowing subscription payments via Stripe. I also need Shopify to provide an API so I can query my user database and do stuff with it.

Writing it on your own is too much trouble. You are responsible for backups, bugs and everything. It's lot more work and it's not even the core feature of our start-up. We want to spend time solving unique problem not redundant one.

VAT compliance, taxes etc... Different kind of reports like churn rate etc... Building all this will require domain expertise and is prone to errors in understanding.

GDPR is also one big headache.

I want someone else to be taking care of user interface stuff. I should be able to pay and get a gorgeous looking site that works well on mobile and desktop, facilitates user signin via oauth, allows me to set up products and prices, lets people purchase my services on a recurring basis, doesn't try to own my user base and lets me write code to access all the relevant user and account and product data.

Is there a SAAS handling user registration, oauth signin and Stripe subscriptions?

I don't want to write all that stuff yet again.