At least two reasons:

- economies of scale no longer work, and you end up doing a custom ERP for your business from scratch.

- your business changes might invalidate your model quickly. You sell through distributors, but open an online shop -- and suddenly your customer is not one of few dozen well-known businesses with a known address and tax number, but user2252 who bought something late at night last night. And you want to understand the needs and behaviors of both.