That's because part of the appeal of options, and stock options in particular, is that they can be converted into real investments even though the options themselves are intangible. The cost of the premium itself depends on many different factors including market sentiment, rumours about the underlying stock or commodity, overall market prospects at the time and other elements. That's because there is less chance the option will be exercised. The more time there is until expiration, the larger the premium, since the chance of reaching the strike price is greater. Call and put options move in opposition.
Call options rise in value as underlying market prices rise.
Put options rise in value as market prices fall.