Don't get me wrong, you can farm during the day, but it's incredible watching people trying to farm up a Maelstrom during the first night by hitting creeps. Semi-carry heroes are all about getting gold and experience by KILLING HEROES and PUSHING TOWERS - in fact, if you're successful you can get far more this way than you ever could from hitting creeps.
The other big issue is the effect this has on the enemy team - DOTA isn't just about gaining gold and experience for your own team, it's about stopping the opposition from getting it. The total amount of gold/xp you gather is far less important than the GAP between your team and opposition.
Match Link
Probably,and most surely,the meta indicates that semi-carries are usually the right-clickers in these short games( Night Stalker in your example),and they take their time in making sure that they will be able to right click properly.
A gank is not always successful,think about it.I've seen countless Night Stalkers initiate on people with Void,using Crippling Fear,then the enemy would just teleport away from him( Bloodseeker treatment).