Why are there so many movie sequels?

Do two half shells make a full shell? Expect more turtle power in Teenage Mutant Ninja Turtles: Out Of The Shadows. Photo: UIP Malaysia

What's with all the movie sequels lately? This year, it seems as though every other movie released in the cinemas has been a sequel.

In 2016 alone, there are at least 20 sequels on the cards, including (deep breath) X-Men: Apocalypse, Teenage Mutant Ninja Turtles: Out Of The Shadows, The Conjuring 2, Finding Dory, Now You See Me 2, Independence Day: Resurgence, Ice Age: Collision Course, The Purge: Election Year, Star Trek Beyond, Jason Bourne, Alice Through The Looking Glass, Mechanic: Resurrection, Bridget Jones’s Baby, Underworld: Blood Wars, Jack Reacher: Never Go Back and Rings.