Good question! For starters, the annual cost of a full-time senior-level developer now exceeds $150,000, 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.
With the monthly plan, you can pause and resume your subscription as often as you need to ensure you’re only paying your developer when you have work available for them.