For me it's because I can't help but think of all the steps as one big wall of a step, and I get overwhelmed. What helps me is to specifically plan to do one tiny thing. Like, I'll plan to put the screwdriver next to the box with the bookshelf pieces in it. That's it, that's all I'll do.

And some days that's indeed all I do, but some days I think "well, while I'm here, I guess I'll also get some of the hardware out of the bag to make sure I don't need to grab any other tools" and then I think "well, I'd better at least install the screws I took out so the cat doesn't play with them and lose them" and then I think "well, I might as well finish off this board with the screws in it," and that continues until the whole thing is done.

If I specifically go in planning to do every step, then I can't help but think about all the work that would be all at once, but when I leave each subsequent step as something I don't even plan to do, then I usually have the motivation to just do one more step.