"Confined" gives the sense of imprisonment. It can be used to say that someone is unable to get out of a situation, and even though a person may need to be in the hospital, it doesn't fit the general feel of "confined." A hospital is a place that people (usually) go voluntarily. Unless there is the case of mental illness, a hospital cannot make a person stay there, so they do not confine people. However, as Gary mentioned people can be confined to bed or wheelchair. And if you do use "confined to the hospital" it will give the sense that the person wants to get out, but is not able to because of health or mental health problems.

'confined' isn't a very good word to use on it's own. It used to be used for women who were pregnant, so that could cause confusion! You can say confined to 'somewhere' e.g. confined to bed, or confined to a wheelchair, if it is somewhere you can't get out of. Perhaps because people don't like the idea of not getting out of hospital, it isn't common to say 'confined to hospital'.