Most companies have a users table, and there are a ton of products that can, without code, ETL a users table into all sorts of tools. There's no redundant work -- in fact, for developers there may not be any work to do at all?
If you want to trigger actions rather than observe them, you need a secure way of authenticating those actions. That's why Clerk does session management :)
And then, connecting the dots a bit, spoofed clickstream events could trigger some sort of marketing campaign event or other "action" that can become a vector for security issues, such as Eve figuring out what Adam has in his shopping cart by manipulating a cart-abandoned campaign to go to Eve's email instead of Adam's?
If so, this is usually addressed by server-side analytics events. I fail to see any mechanism by which this could be solved client-side, nor any useful way for any vendor to make this process meaningfully easier. Analytics events are basically as easy as console.log with most vendors.
Does anybody use ETL-for-marketers tools to extract User data from one source, then load it into Stripe as Customer objects?
I don't think it's common. We mostly see developers creating Stripe Customer objects themselves in their backend.
If I understood your original question correctly, you were asking why ETL tools don't do this? I think it's because these tools are deliberately capable of working with fuzzy data streams, so it at least feels weird to start using them for secure actions. Their security model may also make it dangerous to try.
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Somewhat aside: Authenticating actions from the client is most-easily solved by signing JWTs. Tools like Hasura are doing this in the wild, though it isn't too common among third-party APIs yet. Another close example is Intercom, which asks you to use a non-JWT signature to pass in user data: https://developers.intercom.com/installing-intercom/docs/ios...