Many startups write something that they can sell, get experience, realize what they should have been doing instead, then do that.
UNLESS your team lacks the experience to build and maintain a test suite with a high benefit-to-cost ratio, which is unfortunately the case for most teams.
In this case I don’t have any good solutions, other than choosing one’s team and teammates carefully, before starting the project.
I suspect the reason startups don't do the bare minimum here is because their developers tend to be cheaper and less experienced (at least until they get funding), and "we move fast and break things" is just a lame justification for incompetence.
Product and product research are the same.
I recommend watching Kent Back’s 3X talk for that, I also wrote a blogpost about it: https://rchavesferna.medium.com/designed-v-evolutionary-code...
In practice I've never, ever, ever seen that happen: it's too much upfront cost to start over, especially when you have real customers and real data on top of it and you need to keep pumping out new features.