Most people struggle with walking up to strangers and engaging in conversation.
The fact you don't know whether that person would want to be bothered is the biggest fear.
You can wait for a woman to "show interest" but depending on how attractive you are/versus how picky you are, it could be a while before you get a good opportunity.
It's not rocket science if a woman you deem attractive is eyeballing you. But for a lot of brehs, that doesn't happen, therefore they have to force the issue and put pressure on their conversation skills to create the opportunity.
Also, for some guys, they have to hold their personality back, to not come off as lame. I had to learn through years of trial and error as to what not to say around women.
That's pretty much the sum of it. For example, if I see an attractive woman that I like, but she's not even looking in my direction, I don't even bother, because in my mind, she's not interested. And if she's not interested, then I'm not going to waste my energy or neither of our time trying to spark up a conversation.
My problem has always been either picking up signals or starting off the convo. But once I get introduced, i'm good money.