As a company grows, development becomes harder due to the need to interface with so many teams -- a feature that you used to be able to implement a couple days after a lunchtime discussion with your coworkers turns into a detailed design and design review, design revisions to prevent breaking functionality that other teams use, a revised design review and approval, security review and approval, and then finally implementation, but you find that in the 3 months since you started the design, the functionality you were planning on using was changed by yet another team and the affect on your feature was missed during that project's design review.

Plus, the problems you need to solve get more complicated, you have new compliance needs, paying customers are demanding new features that may require changes across your entire stack, performance is more of an issue (saving 3% of your compute costs didn't matter when you were paying $500/month for infrastructure, but saving 3% of 10 million dollars/month starts turning into real money) and writing and deploying software used by hundreds of millions of people is literally orders of magnitude harder than software a few million people use.