This can have several reasons. I will mention a few.
If the domain has been acquired by a domain investor, they assess that the domain will be worth more in the future than what they bought it for. Domain investors often do not develop their domains, but it can be done. Often they put the domain for sale again on domain marketplaces.
If the domain has been acquired by an end-user, it can take quite a while before the business is started and the website has been developed. These are very expensive projects that many people are working on, and at some point there will be an official launch with a lot of PR involved.