As explained in this article, Company goals typically have a longer duration than Team & Individual goals.
Another important reason for grouping Team & Individual OKRs under an objective cycle is that they will be more in numbers as compared to Company OKRs. The grouping under objective cycle allows you to take collective action on all of them simultaneously. Had they been scattered as independent properties, one will have to operate on them one by one. Active company objectives are going to be small in numbers (not more than 5-6) thus managing them one by one won't be as much painful.
Although we believe above approach right/feasible most of the times, we have added an option to add company objectives in objective cycles in v3.9.5 on request of our customers. Now from v3.9.5 one can add company objectives in objective cycles.