The reason is simple, in the beginning you're starting with no legacy code. Features are simple to add, because there is no history to work around. However, even a month in, features are going to start blending together, and new features will have to work around existing features.
Your databases will get new tables, and the existing tables will get more columns. The response code is going to fork as the API versions increment. The API space is going to grow.
Technology which handled 1-1000 requests per minute will fall over when they start getting hit with 10,000 requests per minute, and require optimizations, indexes, caches, and other complexities in the stack which aren't new features.
Startups are fun to work for early on, when complications like these don't exist. It requires more than "the best engineers" to keep up development velocity, let alone feature velocity.