Immobilized is not Helpless/Restrained
The reason Immobilized doesn't grant advantage is because it merely limits you to one location and does not actively stop you from performing any defense. Even prone doesn't provide advantage if defending against range. The fact is, being stuck in one square doesn't mean your entire body isn't usable, or that your senses are altered just yet. That's where other status effects jump in. Someone tied up is Restrained, and all restrained critters are immobilized but not all immobilized critters are restrained.