You're agreeing with me, the reason the US has two parties is because they barely have any specific identities or policies beyond being left/right. They don't really bother with manifestos, they routinely see people totally at odds with the party elites mount leadership challenges - which can work - and they don't whip, as you observe.

In a system in which parties stand for little, you don't develop third parties. But even in countries with tighter party discipline, like the UK, third parties struggle to justify their existence and stand out from the crowd.