There's a limit to how much moisture silica gel can absorb. Once it's soaked up all it can, then it may as well not be there. You can dry it out in an oven and it will work again quite happily, but the reality is that a small bag will stop working in average room conditions within 15 days. A larger bag will last longer, but even then, not a lot longer.
Within a guitar case, the process is a bit slower but unless it's hermetically sealed (which would involve it having to be very strong indeed to withstand both positive and negative differences on air pressure), the very act of absorbing moisture will slightly lower the air pressure inside the case and draw a bit of external air in. Draughts and small temperature differences will also create a small air change rate within the case, bringing in new air with a higher moisture content, which the silica gel would absorb until it can absorb no more.
I doubt that even if the case was packed with silica gel that it would work for much longer than a year in typical UK room conditions, less in more humid conditions.
The truth is that very few products are packed with enough silica gel to protect them fully from moisture during the shipping from say China to the UK in the winter; it's generally a token gesture.
Yes, you can keep opening up the case and drying out the gel packets, but this isn't a leave and forget strategy. You are better off storing your guitars in a reasonably well temperature regulated space.
Whether you also really need humidity control will depend on where in the world you are. Rarely required in the UK if you have central heating.