It's not really that brilliant. That's just how bookies work
They estimate the realistic odds (what 10000000 / 1 ?) then work out what sort of safety margin they can put around that before offering you your odds. The more outlandish and unpredictable the bet, the bigger safety margin they want.
Hence, they give you a massively unfair and marked up bet based on that (100 / 1)...
If you want to bet the other way around (i.e. that we don't make contact with aliens in the next year), they probably wouldn't take that bet anyway, and if they did, then they're never going to give you 1 / 100, it'd be more like 1 / 1000000000000)
Bookies making money has nothing to do with growing their money with interest.