What can you really build in a weekend?

The core functionality with happy path? Probably, yes.

But will it be useful? I doubt it.

Can you add support for settings so users can customize the workflow in that weekend?

What about error states when things go awry?

What about security?

The mobile or desktop optimized version?

What about compliances needed to serve?

And onboarding, so users can actually get some value out of it?

Admin panel? Customer support tool integration? Handling weird payment flows? Marketing site?

Converting a weekend hacked up prototype, to a proper functioning business takes a lot of time and effort.

Yes, you can come up with a prototype that users can use and play around with, and you should to reduce time to feedback!

But don’t belittle other people’s work of turning prototypes to a proper business with a flippant attitude.