And what do you propose they do with Ashe? Or Olaf, or many other champs who have no dashes, no actual ability dedicated to an actual escape. There has to be something for them, despite it needing to be available for everyone else.
Flash is generally a pick no matter what. The spell is not for mistakes, but for positional play. Juking an ability with Ashe, making the over the wall pick with Irelia, surprise ult from Amumu, who then bandages into the back line to pin down said Ashe. People are so focused on using it for "Covering mistakes" that they really don't actually get what Flash is used for.
I use ghost on both Olaf and Vlad, as they need less instant moves, and more sustained fights. And generally speaking, ghost is better. Almost half the CD, travel much greater distances, and allows for so much more mobility. But it's susceptible to CC, to dashes (Therefore not a reliable escape tool) and since it's not instant, requires more planning for jukes and escapes.
The real problem comes into the fact that despite Flash being an ok spell, it's more prevalent for a couple of reasons.
- Most Pro players have it in their kit at just about any time of the day, therefore leading everyone else to follow suit,
- There is less counterplay to it. Heal gets countered by Ignite, Ignite is a very Niche tool, Ghost requires a lot more planning and can't provide squishies enough time to react, Barrier doesn't do anything about the situation aside from damage mitigation, Cleanse follows suit with Barrier (Though admittedly better situationally)Teleport isn't a combat option, and Exhaust is a single target prevention tool.
This leaves Flash in the unique position of being more versatile than the other spells, which can change throughout the course of the game
Overall, it's not that Flash is overly strong, it's just that the other options aren't up to snuff.