A lot of startups' intentions is to look "serious". If everyone has an engineering playground full of microservices with 5 different programming languages all hosted on AWS, you have to be the same if you want to join the "big boys" club.
Sometimes startups don't even care about solving the business problem. The objective is instead to get the "startup CEO" lifestyle for the connections and reputation regardless of whether you end up solving your business problem in the end. Same for a lot of engineers - they'll happily join and build the engineering playground while enjoying their salary regardless of whether it's the right option from an engineering point of view. If anything, an engineering playground gives you the opportunity to manage lots of people and solve (self-inflicted) problems that will look good on your resume in a way that a boring solution with a handful of engineers won't.
On the other hand, if you live and die by solving the business problem the most cost-effective way, all the "scalability" goes out the window and a simple stack on top of bare-metal makes much more sense, because the only thing that pays you is solving the business problem (in the cheapest way possible) as opposed to bragging about your self-inflicted technical complexity at AWS conferences.