Pro-activity used to help you winning your game.

But nowadays, being pro-active (essentially roaming/ganking) is really DANGEROUS, as the slight mistake will lead your team into an huge deficit, and this since minute 3 into the game.

You can still be pro-active, but it is much more safer to be reactive, as the first mistake from the enemy will likely cost them the first objective (turret, sometimes drake) SO, what is the problem then ?

It has become very hard to be able to express yourself as a player with the current ratio of pro-active play you can do in a game.

Enemy Galio is 6 ? Better not try to do anything. Spellbook Support or Mid + normal TP from top ? Better do nothing. Enemy Varus got Guinsoo ? if you ADC don't have Guinsoo or IE/Statik, do nothing. Kha'zix got Drakthar ? Better not do anything as you literally never see him on the map.

The reasons for the game being in this state are multiple, but principally :

1. First turret gold
2. Amount of teleport (AKA Spellbook) too high
3. Roaming possibilities increased (Movement speed rune), Galio/shen/tf/Tahm kench, there's so much semi global/global on champion nowadays you almost got one in every game.
4. Support becoming a real champion, making snowball in bot lane twice as efficient.

I'm not saying that all those 4 are bad, but they certainly are factor to the un-interactiveness the game as become.