Full stack != learning react.
Craigslist has a great minimalist interface. There's a lot of ways it could go wrong, and it avoids a great majority of them. That said, I don't think a lot of development has gone into actually considering their use cases and developing workflows that prefer them.
Obviously, it's still a very useful site regardless, but I would not call it a well designed site.
If a "proper designer" got hold of it, they'd ruin it.
It is possible to have a successful website without having good front development.
It's possible to create a lot of useful things that don't have good design.
Usefulness is good design.
Good aesthetics are nice when you have them, but are absolutely not a requirement for a good piece software so long that it does it's job and the UX doesn't suck. Most real world people don't care how it looks as much as a lot of designers think they do.
Not too imply design isn't important for a real product. But if you're hiring full stack developers, then you're probably not making a ton of comps.
And on the other side, an app can be aesthetically beautiful and total have crap UX, which is too common with a lot of startuppy products.
Although they have better UI/aesthetics, from a UX perspective, Google and Amazon famously hew pretty closely to their original product design.
What matters to these companies - and, more importantly, their customers - is delivering the content that the customer wants, at the fastest possible speed.