Because a brain gets more valuable the more people use it, and seat pricing punishes exactly that. Heidi charges one flat price for the whole company so there is never a reason to keep someone out.
Per-seat pricing creates a bad incentive: every new user is a new cost, so companies ration access, and the shared brain stays half-empty. Heidi's value comes from everyone reading and writing the same memory, so the pricing removes the barrier.
The variable part, 5 percent of token cost, scales with actual work done, not with headcount.