This is a good question on many levels. Also there are many reasons for this variety of goods and services. Let me give you some of these reasons.
First, from a common sense point of view, there are a lot of people in the world and all people are unique. They, therefore, have different wants and desires. This fact alone creates diversity. And marketers and companies want people to consume. So, they create variety.
Second, there is a psychological reason for this. When there is variety, people feel more in control and they are more apt to buy. So, from a marketing point of view, it might cost very little to have variety, but it can create a windfall in terms of profits.
Third, all competitive industries have many companies vying for more market control. This, too, creates similar products with what appears to be endless variations.