Why are two such similar movies being made at the same time?

Sometimes, people have the same idea at the same time. A few notable times this has happened:

• Tombstone and Wyatt Earp
• Antz and A Bugs Life
• Mission To Mars and Red Planet
• The Abyss and Deep Star Six and Leviathan
• 1492: Conquest of Paradise and Christopher Columbus: The Discovery
• Mirror Mirror and Snow White and the Huntsman
• Armageddon and Deep Impact
• Capote and Infamous
• Dante's Peak and Volcano
• Olympus Has Fallen and White House Down
• Gordy and Babe
• Saving Private Ryan and The Thin Red Line
• No Strings Attached and Friends With Benefits

In all these cases, the movies were released within a few months of each other.

This is a case of what is known as "Twin Films". I'm not sure who coined the phrase exactly but there is a Wikipedia article which details some of the reasons that twin films are made and there is also a pretty comprehensive list of examples.

Some possible reasons that twin films occur as mentioned in the article are:

1. Two or more production companies investing in similar scripts around the same time
2. Industrial espionage
3. The movement of staff between studios
4. The same screenplays are sent to several film studios before being accepted
5. Films dealing with topical issues, such as volcanic eruptions, reality television, terrorist attacks or significant anniversaries, resulting in multiple discovery of the concept

Twin Films

Twin films are films with the same, or very similar, plot produced or released at the same time by two different film studios. The phenomenon can result from two or more production companies investing in similar scripts around the same time, resulting in a race to distribute the films to audiences. Some attribute twin films to industrial espionage, the movement of staff between studios, or that the same screenplays are sent to several film studios before being accepted. Another possible explanation is if the films deal with topical issues, such as volcanic eruptions, reality television, terrorist attacks or significant anniversaries, resulting in multiple discovery of the concept.

Screenwriter Terry Rossio notes that there are always multiple film projects with similar subjects being developed in multiple studios while usually only one of them makes it into production in a given period of time, and therefore twin films are better regarded as exceptions. In one case, for the 1974 film The Towering Inferno, the fear of having competing action thrillers, both set in a burning skyscraper, convinced two Hollywood studios to merge their productions into one (all-star) film.