It's like talking to a wall. Botrk deals Current%HP dmg.
If you think you should be buying it every game, this is wrong.
The highest tier of pro-play isn't 'just every game'.
Botrk scales as an item, almost every person that buys it in pro play is thinking about late game, because if it wasn't pro-play, they would win before late game exists. Not only are they considering the fact that they're scaling out the whole game, but when they buy it Baron normally crosses their mind too.
Combine it's actual mechanics of bleeding off the high poke HP and it's easy to see why it's favored, you just auto 3 times and then use your abilites to nuke their remaining HP and that's enough to kill someone on almost any ADC. Then you also get a crazy slow-speed stealer that is more effective and useful now that ghostblade is better and being spammed and the re-rise of mobility creep.
But if you think this is a cure-all replacement and is better than a 3/0 IE start, nope no way.