They sent me to a rehab center for pt for pelvic fracture. Why can't I go home?

Can you walk? There is usually no attempt to pin or surgically manage pelvic fractures. If you are able to walk, manage at home, do your shopping and self care without too much pain, then probably you do not require in patient rehabilitation.
Its not safe. If you can get up and out of a chair or bed without assistance, dress and feed yourself, get to a phone, and run out of your house if its on fire, you can go home. If you can't do all of these things without assistance, then it wouldn't be safe for you to go home. If you have someone at home that can help, that changes things, but if they can't lift you then you'll need more full time care.