This may be done to hide information from competitors (which may include non-disclosure agreements), or as part of a marketing strategy to manage public image and generate expectations and interest from potential clients.
[1] The phenomenon is well known in the venture capital (VC) community.
Since investors may have to disclose funding a stealth startup, their names are made public; but often, only a general summary description is known about the company.
[2] Startups operating in "stealth mode" often do so to protect new ideas and intellectual property.
[3] If a startup is working on a revolutionary product that would be easy to replicate, operating as a stealth startup and not publicizing product details decreases the risk of the product being copied by other companies, as does having their employees execute NDAs.