We've all seen it in those mobile games: the first level of a skill costs 1 point, and the next one costs 10 points, and the next level costs 50 points, and so on, in some way or another.
I'm here to tell you why that sucks for long term play, and why everyone does it.
The second part is the easiest. Why does everyone do it? It arbitrarily prolongs the playing time of the game, so you will be tempted to spend more time, see more ads, buy more gems, etc. It takes less design work to make since you don't have to assess different strategies against one another.
Why it sucks is fairly straightforward, but takes longer to explain: If all the level 1 skills cost 1, and there are 5 of them, and all the level 2 of those skills costs 10, then you will most likely get all the level one skills first, then all the level 2 skills, and so forth. Specializing doesn't happen, just general heroing. You have 1 path really. Next game, same thing. The same grindy path.
If on the other hand, level 1 and level 2 skills and indeed all level of skills cost 1 point to get to, then once you have bought a level 1 skill, and you get another point for upgrading, you have an actual meaningful choice still: do you specialize in that skill and boost its level, or do you get another skill. There are so many more types of paths for your skills during the game, therefore more interest, more replay-ability in the future, and more strategies for play.