We do not need our bodies once we leave this world regardless of what you think happens after we die. We should be focused on curing diseases and extending the life of living humans. Science would go so far if we used human bodies after death instead of requiring people to give consent to something they don’t need.
Some doctors and scientists are really fucked up and value their experiements over human life. If bodies become a resource they can claim, some patients may not get the care they deserve because the body would be valuable to their studies and experiments.
There is also concerns for the organ market.
Culturally, humans have long standing and many unique traditions for caring for their dead. Someone and their remaining family should not be denied their funeral rights because science wants their body.
A better option would be increasing the amount of awareness for these programs so that people willing to donate their body or organs are informed of their existence and goals and can choose to donate.