Why businesses think it is ok to make me wait once they receive a phone call?

When I visit a car repair shop, car garage, a store, a shop, any kind of small business and I begin talking to the owner or the service manager, why do they think the phone cals they receive have priority to live customers standing and talking to them? I find it extremely rude when I talk to the business owner and then all of a sudden he receives a phone call and starts walking into another end of the office or outside and tgalks to someone for minutes and sometimes for a very long time making me wait. I understand that the caller maybe an important and repeat customer, who maybe buying a service for a lot of money and >I am usually no one who just spoends $20 to $5000 and that is just rubbish to he business owner that they can ignore and let me wait. Sometimes and usually I just leave and never return to such business or I reduce my purchases there immediately upon such treatment. Especially if they don't excuse themselves. Then he quotes me a ridiculously high price after making me wait 15 minutes and listen to his ridiculous conversation with his other customer. From now on, when this happens and don't wait even a second and leave immediately telling the business I will not be back.