A drug-like molecule is built from a small alphabet — mostly carbon, hydrogen, nitrogen, oxygen, and a few others — assembled into a framework with attached groups. Because the same atoms can be connected in enormously many ways, the count of possible molecules explodes very quickly.
Estimates of the number of molecules that could plausibly satisfy basic drug-like rules land around \(10^{60}\). The number of compounds that have ever been synthesized and recorded is roughly \(10^{8}\). The gap between those two numbers is the whole difficulty: the explored region is a rounding error next to the space that exists.
Put the ratio in perspective. If every recorded compound were a single grain of sand, the unexplored drug-like space would be more grains than there are atoms in a large fraction of the observable universe. No amount of faster testing closes a gap of that shape, because testing is linear and the space is not.
This is what makes drug discovery a search problem rather than a lookup problem. There is no index to consult. Any method that only examines molecules someone already made is confined to a tiny island, and the molecule that fits a new target well may sit somewhere in the ocean around it.