It doesn't "sit there until it dies."
It feeds on blood. It swims inside of the penis if you decide to take a piss inside of the Amazon river, because it can sense the ammonia from the urine. It normally attacks the gills of fish, slipping in to eat their blood. When the host dies or blood is otherwise unavailable, it finds a new host.
So, don't piss in the river or a little candiru will find a nice home with plenty of bloodfood in your dick.