No one likes animal trials, most of all the researchers themselves who work with the animals. For example researchers cannot take any vacation during the trial. In fact someone needs to be in the lab at least once a day, including Sundays and public holidays.

Also animal trials are expensive.

Research on alternatives is progressing. It’s not like there is a big conspiracy of sociopaths that get off on animal suffering and want to keep the status quo because of that. It’s simply really really really hard to simulate a body to the necessary level.