How do their teams test code ? When you talk to them about testing, are you talking about the act of testing or who will do testing (dedicated QAs vs Devs/whole team) ?
Some founders might find it expensive to bring in QAs IF their dev team is good at testing and if the rest of the team can help in testing via bug bounties. One reason I heard for not hiring a QA early on is that the product does not have any customers yet and the concept might not even succeed. So, it might be ok to ship a product with “just enough testing” (by Devs).
OTOH, if the founders don’t even expect their devs to testing beyond unit tests and some manual smoke tests, then its another problem. If that is the case, then have you asked them why they don’t want to do deeper testing or postpone it ? I think that would give some clues as to how to motivate them to test deeper.