I don't have a suggestion for why you don't bathe, other than the reasons stated before me. However if you live with someone maybe get them to remind you to do it every few days until you get into the habit (which takes 21 days to form) and then try for every other day, again until it's a habit. Then try for every day. I'm speaking from how I'm starting to do it because bathe to me is beyond and chore that I'd have around the house or anything and I just can't seem bring myself to turn on the water and get it to the right temperature and actually unclothe and then get in and do all the motions there and get out and dry off. Just typing it makes me want to curl up in a ball and lay down for a while to rest. But that's my biggest suggestion.
Something else you might try is making a calendar that you put stars on for everything that you accomplished that day (different colors for different things) or do a chore chart that they have for kids, and write over some of the chores to add your own of what you struggle with doing like taking a shower/bath. Then reward yourself at the end of week for all that you accomplished. Though this may be hard because, from personal experience, when I don't want to bathe I usually don't want to do anything but sleep or lay around. (now I'm starting to add to get on here and write to people because it makes me feel good about myself to share even one small thing to someone that might help.) But if that's your reward, laying down and sleeping or just doing nothing, then so be it until you have the energy to actually do something positive and nice for yourself.
Just two suggestions. Best of luck and wishes to you.
Kelly