Good question! For starters, the annual cost of a full-time senior-level editor now exceeds $100,000, plus benefits (and good luck finding one available). Aside from that, you may not always have enough work to keep them busy at all times, so you're stuck paying for time you aren't able to utilize.
And that's just one job. You'll need a designer and a motion graphics hire as well.
With the monthly plan, you can pause and resume your subscription as often as you need to ensure you're only paying your designer when you have work available for them.