Its a fair question and it came up while I was doing a demonstration of moulding planes, including hollow and round planes, at my local woodworking club. "Why do you work this way?" someone asked, which for the purposes of this post can be rephrased as "Why do you work with hand tools?" This can be a complex question, and one that varies as you ask different folks. I'm going to put my answer here as I've been thinking about it since Saturday.
For me it comes from a variety of places. Since ranking the reasons in some sort of order would be difficult I'm just going to go with whatever order they come to my mind:
Connection with past generations of craftsmen.
When I pull a tool out of my chest, more often then not I'll be picking up an item much older than myself. I'll be wielding a tool that has done this work before. Sometimes I know the name of that individual, sometimes I'm related to them.
No upgrades necessary.
Its rare to not have to upgrade things now days. I like that I own a Stanley #8 plane and I know it has done this work before and will continue to do so as long as it is cared for. I can use this plane, and hopefully my son can use this plane, etc.
Time is the investment, skill is the reward.
As you spend time with hand tools you engage in an ongoing learning process. As you work you learn more about the work you are doing, skills grow in this work and don't become outdated. They skill you acquire is yours forever.
Less Dust and Less Noise.
Requires little explanation other than my shop is in my basement and both are appreciated.
My son can visit my shop when he's a little older and hang out while we do stuff.
I don't do production runs.
If I'm making something I'm usually only making one, so the time economy of machine setup isn't useful to me.
Now to be fair I do use power tools in my work when necessary for savings of effort or time but by and large I just use my hand tools because its pleasant and enjoyable and that's what I want from my shop time.