UNITY_ADS directive disabled

Hello! Just installed 2019.4.4f1 lts version and started a new project. When UnityAds are enabled in Services settings UNITY_ADS directive continue responding with false, code is gray and ignored by the app.

#if UNITY_ADS
//code is ignored even if Unity ads are enabled in the Services tab 
#elseif

Only when I manually insert “UNITY_ADS” in Scripting Define Symbols input in Project settings it starts to work properly.
What are the reasons for this could be?

Maybe the ads, since they likely use native iOS/Android features, are simply disabled in the editor. Did you look at the docs for that API? I am completely unfamiliar with it, so I am merely offering one speculative answer.