A bus will not drop you off right in front of a specialist clinic for your weekly appointment. Also buses are incredibly hostile for wheelchair bound people, even if they are not meant to be. Seriously try boarding a bus during rush hour on a wheelchair - the driver has to help you and people have to move to give space for you, but in general it's an unpleasant and humiliating experience(people who are usually already late are suddenly even more late because of you, and sometimes even comment on it - you need to grow some proper thick skin to not give a damn).

Besides, even if you are on a wheelchair with lower body paralysis you can still drive with hand controls.