Because of the nature of growth when you can only acquire a user once. By definition, the first customers you get will be the most engaged (because they responded first). As you keep hitting people over and over again, conversion rates tend to decline.

Note that this is not always true in B2B, but even there the first few enterprises you sign will probably be better quality than the last few.

Because nowhere in recorded history, at least that I am aware of, has growth consistently followed tremendous gains found at optimistic starts.

If I have a successful small store, and next year I employ 2 helpers, that's 200% growth in size. If the year after that I open a new store, with 4 workers at each, and me just managing the whole thing, that's another year of 200% growth.

If I then predict that in ten years my company will be employing half a million people, I'll be laughed out of the bank.

But that's what you are expecting: linear growth FACTORS.

In the real world, there's actually a limit to how many small stores of my niche type are needed. When you hit the point where there's a Starbucks inside a Starbucks, you may have overreached...