Startups have limited resources, we prioritize the best we can. As a staff engineer it's my responsibility to bring this to my company, but it's not an overnight process.
There's only so much you can do -- making reasonable assumptions, choosing suitable data structures, database indices, testing various workloads, etc. -- in a limited test environment.
You may spend a week optimizing a feature that only 10 people use or that never gets to the point where it becomes an issue. Or you may have something that can't easily be tested at scale, such as various counts or other dynamic data that are determined by complex queries that you may (likely) find you need to cache but don't necessarily know which values will become issues until you start using the system.