If one account is your entire income, then one rule change, payment-processor decision, or mistaken ban can end your business in an afternoon — with no appeal that arrives fast enough to matter. This is not fear-mongering; it happens regularly. Diversification is insurance you build before you need it, when things are going well and it feels unnecessary.
The aim is resilience, not paranoia: if any one platform vanished tomorrow, you would be hurt but not finished. That security also changes how you negotiate and operate — you are never trapped.