Why is sharpening planes and chisels hard, especially for beginners?

Grinding is the key.

I break sharpening into two distinct steps, grinding, which is establishing the primary bevel, and honing, which is using fine abrasives to make the front and back of the blade meet at as a thin a line as possible. Nearly all the instructions you will read on sharpening talk about honing. They usually mention grinding by saying, “You should use such and such wheel and a good tool rest.” But often an accurate grind is key to the rest of the process. Without an accurate grind, you spend a ton of time at the finer abrasives trying to get a good edge.

It turns out that I think honing is the easier skill. Well, honing is easier once you know how to grind. Grinding is hard because it is hard to find a method that is both fast and easy to control.

Last sharpening impacts current sharpening.

So you just finished reading all the books and articles you can find on sharpening. You pick a sharpening method, buy all the equipment, and head out to the shop to give it a try. Your first attempt does not go so well, so you try again. Well guess what, once a blade is messed up from sharpening, it is sometimes time consuming to restore it to a good state. (This gets back to grinding.)

Sharpening is metalworking, not woodworking.

I know this sounds obvious, but I think we forget it sometimes. Metalworking requires a much higher degree of precision than woodworking. It also involves a material that is much less forgiving than woodworking.