10ft ladders are more common than 10ft poles.
I can think several places in my town where I could buy the ladder, with many options on style, materials and quality.
I don't know of any place that would have a 10ft pole in stock. Most lumber is in 6ft or 8ft sizes, most paint poles would be 4ft to 6ft, aside from expanding pruning poles saws, that 10ft pole would be special order.
That’s because modern ladders have to conform to OSHA requirements and other safety standards. If you look at a modern ladder, even the most simple (and cheap) one available at a national chain hardware store, you’ll find that it is most likely reinforced by steel bars, held together by steel rivets, and quality tested to ensure that it can withstand the advertised maximum load.
On the other hand, the type of ladder one imagines would be commonly found in the average D&D general store would literally consist of nothing more than two ten-foot-long sticks (read: poles), some wooden rungs (little poles), and a handful of nails (not necessarily good nails).
So while it would take a considerable expenditure of time and energy to disassemble a modern ladder (even with modern tools), it would take no more than two or three minutes to tear apart a medieval ladder, with nothing more than a hacksaw or hatchet.
Keep in mind that (in my campaigns, at least) there is no D&D equivalent to OSHA. Who exactly is going to be enforcing 10ft pole quality regulations? If a 10ft pole is too low-quality to be usable, then it should be obvious (filled with knotholes, rotting, etc.). That’s not something a PC should find out when he’s climbing across a bottomless pit. He shouldn’t have to ask if the pole is constructed to meet Waterdeep Builder’s Association code.
As others have already stated, it would be immensely useful if 10ft poles came with extra features, like reinforcement, or joints so they could be collapsed. However, those things need to be specifically added to the item description. Otherwise, DMs will assume they aren’t part of the normal item. In the 3.0 Arms and Equipment guide, such poles cost extra. They cost an inordinate amount, but still more than the standard pole. I think the standard price is fine for a 10ft pole that collapses to two to four feet in length.