Many bicycle panniers are made of waterproof material. Backpacks are not, necessitating built-in or separate raincovers. Why don't backpacks use the material that waterproof panniers do? Unlike gear to be worn on the body, there's no need for backpacks to breathe.
There are quite a few backpacks made of waterproof materials, especially among cottage manufacturers. ZPacks, Hyperlight Mountain Gear, Zimmerbuilt, Gossamer Gear, and many others manufacture packs out of Hybrid Cuben Fiber, Dimension Polyant X-Pac fabrics, or other waterproof materials. Even more mainstream manufacturers use a good deal of waterproof fabrics. Speaking anecdotally, my REI Flash 45 uses waterproof fabric for most of the pack body.
However, there is more to waterproofing a pack than the fabrics. Even packs that use waterproof textiles are not sufficiently waterproof unless the seams are sealed. Moreover, water can enter through zippers or drawcord openings. There are some packs on the market designed not to need a liner or cover (the ZPacks Arc Blast, for example), but achieving this requires a number of design considerations. They must simplify construction to minimize the number and complexity of seams to seal, they must eliminate features that create entry points for water (like hydration hose ports and zippers), and they must use a rolltop closure to seal out water.
On a simple ultralight pack, this isn't much of a compromise. But as complexity increases, rainproofing becomes more and more difficult to achieve, to the point where it is simply easier (and lighter) to add a pack liner or cover.
The complexity of backpacks, as well as the typical use case scenario has a lot to do with why the backpacks themselves aren't waterproofed.
For example, the typical day in the life of a bicycle pannier involves relatively little exposure to water. You take it out in the rain for an hour or two, and then usually you take it inside with you wherever you go. This makes waterproofing the pannier itself logical, because the waterproof layer can sustain those two hours. But waterproof layers typically have an unspoken time limit. If you've ever exposed a 'waterproof' pannier or 'waterproof' boots to water for more than a few hours, you start to realize the limits of waterproof layers. They're usually, not truly waterproof.
Add to this the fact that waterproof fabrics are typically heavy and they don't breathe very well, and you've got a recipe for inefficiency. If you've ever worn a waterproof backpack, you know how heavy they can be and how poorly they breathe.
All this to say, it's more efficient, lighter, and cheaper to place individual items in dry sacks. The lightweight dry sacks. They keep your stuff organized, and they keep things dry no matter how soaked your backpack is.
Backpacks are primarily designed to be carried out therefore they need to be light. They also need to be tough so they can endure the trips the specified weights for a long time.
Although we do get rained out every now and again no one plans a trip around raining days (Oh look, it's raining let's go hiking). So it's more "efficient" to keep the packs light using a lighter material and just put a simple rain cover in case of rain than make the packs heavier with waterproof material.
There are waterproof packs in the market. I own a couple myself.
My petzl portage 30 looks more like a drybag with shoulder straps but I use that one specific for canyoning. I also have a my hiking/trekking pack that uses c-canvas that is waterproof but then my pack weights 3kgs alone. Cactus Foray
My wife has an Osprey Ariel 50 and it's a whole kg lighter than mine. Even if we compare a pack the same size as mine (70L), the Osprey one still a good 800gr lighter and that includes the cover that comes with the pack.
So, in summary...
- yes, there are packs in the market with waterproof specs.
- they varies depending what you are planning to do.
- Having waterproof packs might not be the best approach in some cases. (My wife would hate to carry my pack even to go down to the fish and chips shop.)
Like a lot of decision made by a manufacturer it's a case of profit! At some level they've made a commercial decision. Waterproof fabrics are expensive, so if you make a bag using them then you have pass this onto the customer or take a hit on your profits. I would imagine that they've researched this and decided the demand for this doesn't match the outlay they'd need to invest in such a product.
I've been backpacking for over 40 years now. Nearly all backpacks start with waterproof fabric. The problem with "waterproof" backpacks is primarily the seams, then secondarily the strain put on the various fabrics. The seams are made up of a series of holes poked through the waterproof fabric with thread poked through them. It invariably makes a waterproof fabric not waterproof any longer. Seam sealing to re-seal the holes helps, but it isn't always perfect.
Now consider that every time you load, open, unload your pack, you are wearing the waterproof coating down. Eventually it will wear through and leak. Another problem is that water is pretty invasive. it drips and finds its way into the pack itself. If it's windy, the rain seems to defy gravity.
The best solution is to determine which pieces of your gear absolutely have to be dry, then put them in waterproof bags inside the pack. Don't count on the backpack even with a pack cover, to keep everything dry. Bag your sleeping bag and clothes. The rest can usually survive water...