Because, effectively, you’re denying the duck test
The duck test is a form of abductive reasoning, usually expressed as “If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.”
No-one is asserting that an account that is being played for 18+ hours a day for weeks / months / years on end is account sharing and/or botting, but it sure does look like an account that is and therefore probably is. That’s why it should be investigated to see if it’s actually a duck…