I once read an argument that went something like this: if you're actually going to go to the trouble to build something, its expected return to the business should be so large that how long it takes (within reason) shouldn't matter. Conversely, if a project is only economic to undertake if it can be done within a certain amount of time, it shouldn't be done at all.

I find that I often don't have a good feel for how long something is going to take until I'm about 1/4 the way through the project. If I absolutely had to produce accurate, high-confidence estimates, I would just do 1/4 of the work first, then estimate the rest.

As to why estimating is so hard, here's how I like to think about it. Tasks form a tree: each project has a number of steps, each of which itself breaks down into sub-steps, and so on. We tend to estimate by thinking about the tasks near the root of the tree; but the actual time required is proportional to the number of leaves -- tasks we can implement without further breakdown. Without planning the entire project out in detail it's hard to know what that number is. But to plan to that level of detail requires making many design decisions; and researching and making those decisions is much of the work of the project.

The bottom line is, good estimates are expensive to produce, and my experience aligns with yours: it's rarely worth the trouble.