Honestly, it is not their fault. If they don't stay on you, they will catch hell from the general manager. Also, most customers are not good about making sure they talk to the sales person that greeted them.
Say your sales person decides to give you space and then five minutes later, you want to look at something a bit closer. What happens then is typically the customer grabs whoever is nearby. If they buy, that original sales person gets nada.
Toyota is different. They don't negotiate price and that creates a different dynamic.
And here at my job, my clients are mine. No one else can touch them and if they do, the comission is mine. When you're talking an average commission of $1100 per sale, that's pretty significant, so no one wastes any time chasing other people's leads.