The real reason it takes time is because we try not to harm people even in experimental drug testing. It would be much faster to simply toss shit at the wall and see what sticks, but that's not exactly humane. So we have to find analogues that hopefully mimick humans will enough, but they don't really work well. So it takes lots of time to build up enough evidence with those preliminary tests to convince the safety board to allow human trials. Then trials have to slowly scale up to limit the amount of people harmed by unforseen effects with a lot of time between as the safety board reviews the previous results before allowing the next test.
It's all good to do, but it does make development frustratingly slow sometimes. Especially when people are actively dying waiting for the new drugs.