Because despite his lack of emotions and other human traits, Data is a person, not a service provided by a for-profit corporate entity. He doesn't do La Forge's job for him, nor is La Forge or any other Enterprise crew member expected to perform all their duties with him as an intermediary. He doesn't undermine the competence of every single crew member on the ship; that job was left to Wesley Crusher. (whoops did I just say that out loud) But most importantly, it's fiction... and the writers can contrive to present problems and challenges that Data cannot resolve whereas one of the other characters can. It's also telling that the only truly sentient robots on TNG were the Soong-type androids—quirky side projects of a sort of mad scientist, rather than massive corporate or institutional undertakings. That made them rare and special, and allowed them to dodge the sticky wickets presented by having that sort of AI be ubiquitous.