They don't fully occupy the same niche. Yes, they both give mobillity, but in very different ways, just how Yis ult and Zeds ult are very different in how they give mobillity.
Ghost is(or was-,-) pretty undervalued on champs with extendet fighting patterns, which mostly is tanky dps and battlemages. Champs that don't want to get in range once, but stick there, as well as champs that can afford to take some beating so they don't have to get out of dangerzone(especially cc range) immediately but can survive through the first burst/cc and then run away.
I used ghost alot on juggernauts and bruisers top, and it always felt worth it - mostly because with boots of lucidity+distortion, both ghost and tp got a really low cd for summoners and ghost actually delivered. I often got kills early cause i could close the gap with ghost, burn a flash and still have ghost running to catch up once more, cause ghost covers far more ground total tha flash(unless theres a wall nearby, and the shorter cd than flash also is a big plus.
Without distortion however, neither the boost it gives nor the cd are enough to justify it over flash.
If they would reduce the cd or up the ms to 30% over either a longer duration or with 30-50% slow reduction, it would be worth takeing once more. But as it is, it was fully dependent on distortion to really keep up with flash on those that synergized with with it.
The aim shuldn't be to make ghost balanced for all, but atleast a solid option for those that synergize with it, and that simply isn't the case right now with istortion gone.