No ones making turrets

It's all about balance and what you want to invest in...
Investment on turret - better defense (not necessary true all the time as certain maps it can be taken out by Sniper easily)
Investment on yourself - better offense (so you can push all the way to their base)

What is the point of buying turret when half of the time all your pros & bots are at the enemies' base? Build the right turret at the right time.

Best offense is the best defense. Turtling is not something you want to encourage in any game...